are you going to overclock it? if so, how high do you expect to be able to go?
do you want a single stick 512, a dual-channel 512 (2 x 256), or do you maybe want a gigset? what kind of timings are you looking to run it at?
are you using the computer for gaming, number crunching, or what? there are a lot of questions that need to be asked before anyone can give you any decent help.