Currently I have 2 512 sticks I looked them up on the kingston website, and this is the info I found:

(I only have 1gb left in this comp because my brother bought 2gb, and when he moved out he took the 2gb with him for his other comp.)
I am looking on ebay for 2gb of ram, because its cheapers, and I dont care if its used...
So is there anything special I have to look for? Does it have to be 800mhz? I know it needs to be DDR2, does the voltage need to be the same?
What do you guys think of this? It looks almost the exact same?
(PS I have 4 slots for ram) Hmm thinking about it, I may just buy 2 of those, and have 4gb in my comp... Unless I leave the 1gb in, but I dont think my OS will use 5gb... I am running a 32 bit windows xp...