you don't NEED more memory just because you go 64 bit. you CAN use more of it (>4 GB) than you can on 32 bit. for DDR2, there's not much reason to go beyond 800, and your motherboard doesn't support faster anyway. if you want to add more, pretty much any 2x1 GB or 2x2 GB DDR2-800 set would do. generally, ones with heat spreaders or named 'series' like "XMS" or "ripjaws", etc, indicate higher quality. but if you're not doing anything stressful with your PC, you can really go as cheap as you like. not that there's terribly much difference in price between "value" and "performance" RAM at that level. few bucks maybe.
@bloodeagle, his mobo is DDR2.