the general guide is this:
Get a good name brand: OCZ, Corsair, Kingston, Mushkin
Get the fastest speed you can: DDR 466 or DDR 500 if your overclocking.
Get the lowest latency you can, the lowest latency you will ever find for DDR is 2-2-2-2.
Get matched pairs for Dual Channel, it just is more reliable and easier to manage.
Core 2 Duo E6400, DFI Infinity 975X/G, 2x 512mb DDR2 667mhz, Albatron 7900gt, WD 200gb SATA, Samsung DVD-RW, Silverstone ST-50EF 500w PSU.