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When upgrading an older motherboard, it is sometimes impossible to match the installed memory. Some motherboards have 30-pin and 72-pin SIMM sockets, and many have both SIMM and DIMM sockets. If the installed memory occupies all available sockets of one type, you may have to add memory of a different type.

If so, consult the manual to determine supported configurations. For example, many 30/72-pin motherboards provide four 30-pin and two 72-pin sockets, but allow you to populate both 72-pin sockets only if no memory is installed in the 30-pin sockets. Similarly, motherboards with both SIMM and DIMM sockets may allow using a DIMM only if one or more SIMM sockets are vacant or populated only with single-sided SIMMs.

Mixing memory types may degrade performance. Some chipsets run all memory at the speed of the slowest module. Others, including the 430HX and 430VX, run each bank at optimum speed. Mixed-memory configurations are nonstandard and best avoided. Memory is cheap enough that it's often better to use all new memory.

Be very conscious of costs when upgrading older systems. Although older memory types are still available, the cost per MB may be very high. For the same cost as a large amount of old-style memory, you may literally be able to purchase a new motherboard, processor, and the same amount of modern memory. If for some reason we need to upgrade memory in a SIMM-based system, we try to scrounge compatible SIMMs from hangar queens rather than buying new modules.

Observe the following general guidelines when selecting memory:

• Verify your motherboard can cache the full amount of RAM you install. For example, the Intel 430TX chipset supports 256 MB of RAM, but caches only 64 MB. Adding RAM beyond 64 MB actually decreases system performance. Some motherboards have insufficient cache installed to cache the full amount of cacheable RAM supported by the chipset. Some of these have sockets that you can fill with additional cache. The cacheable RAM area of Slot 1 systems is determined by the processor itself. The L2 cache present on Pentium II CPUs can cache a maximum of 512 MB of system memory. Modern socketed processors from Intel and AMD can cache more main memory than can physically be installed.

• Purchase only name-brand memory, especially if you overclock your system. Motherboards vary in their tolerance for different brands of memory. Some motherboards accept and use nearly any compatible memory, but others are reliable only with some brands of memory. A name-brand memory module functions properly on nearly any motherboard designed to accept that module. We use only Crucial memory (http://www.crucial.com) and Kingston memory (http://www.kingston.com) in our systems, and have never had a problem with them.

• Don't attempt to save money by mixing newer memory with older memory. For example, if you buy two 32 MB PC133 SDRAM DIMMs for a system that currently has one 16 MB EDO DIMM installed, you may be tempted to use all three DIMMs for a system total of 80 MB rather than 64 MB. That's usually a mistake. Some systems accept mixed EDO and SDRAM memory, but most systems will run the newer, faster memory at the same speed as the older, slower memory. The same holds true for upgrading newer systems. For example, if you buy two 256 MB PC2700 DIMMs to upgrade a system that currently has one 128 MB PC1600 DIMM, settle for 512 MB of pure PC2700 memory rather than going for 640 MB of mixed-speed memory that will operate at only PC1600.

• Buy one larger-capacity module in preference to two smaller ones of the same total capacity. This is particularly important with DIMM modules. Most motherboards have only two to four DIMM sockets, so buying smaller-capacity DIMMs may later force you to replace existing memory when you want to add more memory to the system.

• Memory modules and sockets may use tin or gold contacts. Manufacturers often recommend using modules with the same contact metal as the sockets. In theory, mixing gold and tin can cause corrosion and other problems. In practice, we've never seen this happen.