These systems are excellent candidates for memory upgrades. Early Pentium II systems often have only 16 MB of RAM. In the price-sensitive consumer Celeron market, many early systems shipped with only 16 MB, and some Celeron systems have been sold with only 8 MB. Expanding memory to 128 MB or more is the most cost-effective upgrade you can make. When upgrading memory in one of these systems, note the following issues:

• Some early Pentium II and Celeron systems use EDO SIMMs or DIMMs, but most use 3.3 volt 168-pin unbuffered JEDEC SDR-SDRAM DIMMs.

• Conserve DIMM sockets. A few motherboards have four DIMM sockets, most have three, and some low-end systems have only two. If you have the choice, always install one larger DIMM rather than two smaller ones that total the same amount of memory. Note, however, that older systems may not recognize large-capacity DIMMs or those that use 128-megabit or larger memory chips. In that situation, a BIOS upgrade may help, but the limitation is often hardcoded into the chipset.

• Most of these systems have nonparity memory installed, but can use either parity or nonparity DIMMs interchangeably. Unless you plan to install 512 MB or more, install nonparity DIMMs. We have been told that when using very large amounts of memory—more than 512 MB—memory errors introduced by cosmic rays make it worthwhile to pay the additional cost for parity/ECC memory and to accept the small performance hit that using ECC produces.

• Memory must always be added in full banks. These systems use a 64-bit memory bus, and therefore require adding 72-pin SIMMs in pairs. DIMMs may be added individually.

• Most SIMM-based systems use 60 ns or 50 ns memory. Make sure memory you add is at least as fast as the memory that is already installed. Early DIMM-based systems use FPM or EDO DIMMs, which you should match as closely as possible. Later DIMM-based systems may use JEDEC (PC66) SDRAM. You can install PC66 SDRAM, but it usually makes more sense to buy PC133 SDRAM for these systems because it can be recycled later if you upgrade to a faster system.