The processor you choose determines how fast the system runs, and how long it will provide subjectively adequate performance before you need to replace the processor or the system itself. Buying a processor just fast enough to meet current needs means that you'll have to upgrade in a few months. But processor pricing has a built-in law of diminishing returns. Spending twice as much on a processor doesn't buy twice the performance. In fact, you'll be lucky to get 25% more performance for twice the money. So although it's a mistake to buy too slow a processor, it's also a mistake to buy one that's too fast. Consider the following issues when choosing a processor:

Horizon

What kind of applications do you run and how long do you want the system to be usable without requiring an upgrade? If you run mostly standard productivity applications and don't upgrade them frequently, a low-end processor may still be fast enough a year or more after you buy it. If you run cutting-edge games or other demanding applications, buy a midrange or faster processor initially, and expect to replace it every six months to a year. But expect to pay a price for remaining on the bleeding edge.

Hassle

Do you mind upgrading your system frequently? If you don't mind replacing the processor every six to 12 months, you can get most of the performance of a high-end system at minimal cost by replacing the processor frequently with the then-current midrange processor. In the past, this was easier with AMD processors because AMD has used Socket A for years and had standardized on 100/200 MHz and 133/266 MHz FSBs. It was sometimes possible to install a current processor in a two-year-old motherboard with only a BIOS upgrade.

Intel made things much more difficult, replacing Socket 370 with Socket 423 and then Socket 478, and introducing faster FSB speeds frequently. Although many considered these changes as cynical planned obsolescence, in fact these changes resulted simply from Intel's much faster product development cycle. The situation is different now. Intel has stabilized around Socket 478 and the 800 MHz FSB (although the forthcoming Prescott processors will use a different socket), and AMD is in a state of flux. AMD recently introduced the 166/333 MHz and 200/400 MHz FSBs for the Athlon, which will rapidly render older motherboards obsolete. Also, AMD has deemphasized Athlon product development in favor of its forthcoming Hammer-series processors, which are entirely incompatible with the Athlon series. On balance, Intel actually offers a better upgrade path for now, although that may change depending on the decisions AMD makes with regard to Hammer-series processors.

Trade-offs

If you're working on a fixed budget, don't spend too much on the processor to the detriment of the rest of the system. Instead of spending $300 on a fast processor and compromise on the other components, you're better off spending $150 on a midrange processor and using the other $150 to buy more memory, a faster hard disk, and better video. A low-end Pentium 4 with lots of memory, a fast hard drive, and a good video adapter blows the doors off the fastest Pentium 4 with inadequate memory, a slow hard drive, and a cheesy video card every day of the week. Don't make yourself "processor- poor."

Form factor

Keep form factor in mind when you're shopping for a processor, particularly if you're also buying a motherboard:

Socket 7 Don't consider buying a Socket 7 processor, even as an inexpensive upgrade to a working system. Any money spent on Socket 7 is wasted. Retire the old system to less-demanding duties, and build or buy a new system instead.

Slot 1 Slot 1 was obsolete by the end of 2001. Although new Slot 1 processors remain in limited distribution, new Slot 1 motherboards are now almost impossible to find. An existing Slot 1 system may or may not be a good upgrade candidate depending on the motherboard characteristics. Some Slot 1 motherboards support fast Pentium III processors, and can be upgraded at reasonable expense. For example, we recently upgraded an older Pentium II server to a Pentium III using a salvaged processor. Because we used a relatively slow Pentium III processor, even if we had to buy the processor, the total upgrade cost would have been about $75. Performance more than doubled, which gives that server another two years or more of useful life.

Other Slot 1 motherboards have neither BIOS support nor adequate VRMs to support faster processors. Although it's possible to upgrade those systems with marginally faster Slot 1 processors, doing so makes no economic sense. Before you upgrade any Slot 1 system, check prices carefully. Some Slot 1 processors are very expensive relative to the performance boost they provide. You may be able to replace the motherboard, processor, and memory with Socket 478 Pentium 4 or Socket A Athlon components for little more than the cost of the Slot 1 processor alone.

Slot A Like Intel processors, AMD Athlon processors were originally produced in slotted versions, which were subsequently replaced by socketed versions. Slot A motherboards and processors are now almost impossible to find, and any Slot A motherboard is now so old that it is a poor upgrade candidate. If for some reason you must replace the processor in a Slot A system, pay careful attention to the chipset it uses. Motherboards based on the AMD-750 chipset can use Slot A processors based on the K7, K75, and Thunderbird cores (although Slot A Thunderbirds are difficult to find). Motherboards based on the VIA KX133 chipset are incompatible with Slot A Thunderbird Athlon processors, but can use Athlons based on the K7 and K75 cores. As of July 2003, Slot A processors are still in limited distribution, but soon the only alternative will be the used market

Socket 370 As of July 2003 Socket 370 is moribund. Intel pulled out all the stops to push the Pentium 4 at the expense of its sixth-generation Celeron and Pentium III processors, and by mid-2002 Socket 370 was no longer a mainstream technology. Intel still offers a limited selection of Socket 370 Celeron and Pentium III processors. Alas, Intel no longer makes Socket 370 motherboards, so third-party motherboard makers are now the only source for new Socket 370 motherboards.

Although it no longer makes economic sense to build a new Socket 370 system, existing Socket 370 systems may be economically upgradeable. When upgrading an older Socket 370 system, verify compatibility between your motherboard and the Socket 370 processor you propose to buy. There are many incompatibilities between older motherboards and newer processors. Some problems can be solved with a simple BIOS update, but many are unsolvable because the older motherboard's chipset or VRMs do not support newer Socket 370 processors.

Socket A In the past, AMD did a much better job than Intel at maintaining backward compatibility. Intel changed sockets and FSB speeds frequently, but AMD just kept using Socket A and the standard 100/200 and 133/266 MHz FSB speeds. The Hammer-series processors, due later in 2003, will change that, but Socket A motherboards and processors will remain available for at least the next year or two. As long as you don't mind buying into an obsolescent technology, Socket A remains a good choice for a new system until Hammer-series processors and motherboards become inexpensive mainstream products.

Older Socket A systems may or may not be good upgrade candidates. In general, older-model Socket A motherboards can use newer Socket A processors, although perhaps not the fastest models. A Socket A system that supports only the 200 MHz FSB is probably too old to be economically upgradeable. For such systems, replace the motherboard, processor, and memory with current products. Most Socket A systems that support the 266 MHz FSB or higher and that support at least PC2100 DDR-SDRAM are excellent upgrade candidates. By replacing an older Duron or Athlon processor with a current low-end Athlon, you may be able to double system performance for much less than $100. Before you make such an upgrade, verify that your motherboard supports the specific processor model and speed that you plan to install. You will probably need to upgrade the BIOS as well.

If your goal is to build a dual-processor system, your best option is a pair of Socket A Athlon MP processors running in an AMD-760MPX based motherboard. As always, an older motherboard may have BIOS or VRM issues with newer processors, so you still need to verify compatibility.

Always verify the cooling requirements of a replacement processor. The existing CPU heatsink/fan unit may fit the new processor, but that's no guarantee that it is adequate to cool the new processor adequately. We almost learned that the hard way. In late 2002, AMD sent us a preproduction sample of its new 333 MHz FSB Athlon 2600+, including just the bare CPU. We verified that the ASUS A7N8X Deluxe motherboard supported the 2600+, but we didn't think about the heatsink. We'd already squirted thermal goop onto the processor and were about to install an off-the-shelf heatsink when we remembered that we'd gotten in some sample heatsinks from DynaTron, and decided to try one of those. That was fortunate because as we were reading the DynaTron literature we realized that the heatsink we were about to use was rated only for XP 2000+ and slower Athlons. If we'd installed that heatsink and powered up the system, our shiny new 2600+ processor might have burnt itself to a crisp in seconds. Processors aren't much good if you let the smoke out.

Socket 423 Socket 423 was Intel's first socket for the Pentium 4, and was simply a stopgap solution that allowed Intel to bring Pentium 4 processors to market quickly to compete with the AMD Athlon on clock speed. Socket 423 processors and motherboards are obsolete. Socket 423 motherboards are nearly impossible to find, although Socket 423 processors remain in limited distribution. A Socket 423 system is a poor upgrade candidate because the fastest available Socket 423 processor will be little or no faster than the processor already installed. Replacing the motherboard, processor, and memory is a far better solution.

Socket 478 A Socket 478 processor is the best choice if you are building a new mainstream system. An existing Socket 478 system can easily be upgraded simply by dropping in a faster Socket 478 processor, a condition that is likely to remain true for some time. As always, it's possible that BIOS, chipset, and VRM issues may restrict the speed of the fastest Socket 478 processor that can be installed in a particular motherboard, but Socket 478 currently offers the best options for future upgradability.

When upgrading a system, the existing motherboard determines upgradability, as follows:

Socket 7 and earlier motherboards

These motherboards are simply too old to upgrade economically. We recommend retiring such ancient systems, or discarding them entirely.

Slot 1 motherboards

Slot 1 Pentium II and Celeron processors remain in limited distribution, although we expect them to disappear entirely by the end of 2003 or early 2004. Fortunately, some Slot 1 motherboards can be upgraded by using a slocket adapter, which accepts a Socket 370 processor and plugs into the motherboard Slot 1. The best candidates for such upgrades are motherboards designed for the Pentium III that support the 100 MHz or 133 MHz FSB. Even if a particular motherboard can be upgraded via slocket, it may be limited by BIOS, chipset, or VRM issues as to which particular Socket 370 processors are usable. In general, FC-PGA Celerons are the most likely to work, assuming that the motherboard supports the Celeron L2 caching method. An FC-PGA Coppermine-core Pentium III may or may not work, depending on the particular slocket/processor combination and the chipset and BIOS configuration of the motherboard. We know of no slocket that allows FC-PGA2 Celerons and Pentium IIIs to be used in Slot 1 motherboards. Before you attempt to upgrade a Slot 1 motherboard with a slocket, verify with the slocket maker that the slocket, processor, and motherboard you plan to use are compatible.

Slot A motherboards

Slot A processors are now almost impossible to find new. Slot A motherboards are now so old that it makes no sense to spend money upgrading them. Instead, replace the processor, motherboard, and memory with current products. You can buy a decent Socket A processor, motherboard, and memory for less than $200, which makes messing around with an obsolete processor and motherboard a complete waste of time.

Socket 370 motherboards

Upgrading a Socket 370 system should be easy. Unfortunately, it often isn't. The problem with upgrading Socket 370 motherboards is that there have been so many variants of the socket itself and processors intended to fit it that determining compatibility can be difficult. Any Socket 370 processor physically fits any Socket 370 socket, but there are actual pinout differences between early Socket 370 sockets and processors and later versions. Late-model Socket 370 processors—Coppermine- and Tualatin-core Celerons and Pentium IIIs—will not operate in early-model Socket 370 motherboards, and early-model Socket 370 processors—Mendocino-core Celerons and Katmai-core Pentium IIIs—may or may not operate in later-model Socket 370 motherboards. In addition, chipset issues are important with Socket 370 because early Socket 370 chipset revisions do not support later Socket 370 processors, even though the processor is otherwise compatible electrically and physically with the socket. Intel rationalized this situation in late 2001 by introducing its so-called "Universal" Socket 370 motherboards, which can accept any Socket 370 processor. If you intend to upgrade the processor in a Socket 370 system, the best advice is first to determine exactly what motherboard you have (including revision level). Once you've done that, visit the motherboard maker's web site and read the technical documentation to determine which currently available Socket 370 processors can be used in that motherboard.

Socket A, Socket 423, and Socket 478

Motherboards that use any of these sockets can be upgraded using current processors. Socket 423 is a poor upgrade candidate because only relatively slow processors are available for it. Socket A and Socket 478 motherboards are generally good upgrade candidates because there are numerous models of fast, inexpensive processors available for both of them. As always, check the documentation for the motherboard to ensure that it supports the type, FSB speed, and clock speed of the processor you plan to install. Ordinarily, such upgrades are relatively straightforward, requiring a BIOS upgrade at most.