Hi This is my first post, And ram Is a good place to start.
First off if you have a dual channel board then buy a set of 2x512 and install them into the dual channel slots on the board (usally the color code will tell you what slots to use) Then sell the 512 you have now. Resent benchmarking tests have proved that going over 1gb or ram is a waste and actually can hurt the performance of the rig. You waste to much cpu ussage cache'ing the ram. Unless you are doing alot of rendering (cad, video editing etc) Stick to 2x512
The other thing alot of people get confused on is the spead rating if your ram isn't running in dual channel then you will never see 400mhz effective. pc3200 is 100mhz x 2instructions per cycle = 200mhz in dual channel then its 200mhzx2 = 400mhz effective. (very, very, very misleading)