I think if you have 1 gig of ram and put 2 512`s in you lose the dual channeling, so either go for 2 sticks of 1 gigs or go for 4 sticks of 512 which is what I did for my pc and it turned out cheaper.
I think if you use four sticks it will no longer run in dual channel... But im not sure since my system can't use Dual Channel and I don't bother with it any....