In most cases, standard non-parity, non-ECC memory will work just fine. Most boards today are still using SDRAM, although DDR-DRAM has really caught on and is a lot faster. In short, though, memory is not a huge issue and just buy what your motherboard requires. And, with today’s prices, buy lots of it. Operating systems themselves require large amounts of memory. Windows XP’s bare minimum requirement is 128MB of RAM. So, give yourself ample breathing room and don’t try to save a few measly bucks by not getting enough memory.