|If your going to use light-weight applications(word processor, browsing, music) then 2gb is enough.|
For heavy task u need 4gb or more.
I think it's better to buy dual channel kit(2 x 2gb), they are cheap.
If the board use ddr2 then grab ddr2 800 1.8v and for ddr3 grap ddr3 1333 1.5v.
We can not fight new wars with old weapons, let he who desires peace prepare for war - PROPHET.