It can use up to 4 2GB modules (for a total of 8GB max) of up to 800Mhz DDR2 RAM.
G.Skill, Corsair, Crucial, and Kingston are some good brands to look for.
This is expensive (as is ALL DDR2 RAM at this point), but this is just an example. G.SKILL 8GB (4 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Quad Kit Desktop Memory Model F2-6400CL5Q-8GBPQ
Of course you don't have to max it out, you can just go for a 2x2GB kit as well. Something like this. G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model F2-6400CL5D-4GBNT