If you have an A7N8X and a 2500+, you don't need PC3200. The 2500+ is only a 333Mhz chip, so you'll only need PC2700. Why waste money?
I run with Kingston HyperX PC2700. It's their CAS2 512MB stick. It's FAST. This is the fastest system I've ever owned, and faster than my roommate's 2500+ with a gig of 2700 CAS2.5. Mine starts to bog sooner on really heavy loads, but for load times, windows boot-up and the like, my system creams his.
http://www.newegg.com/app/ViewProduc...144-104&depa=1
That's what I've got, and that's what I recommend.
For the AMD64 chips, you need a special motherboard. To further compound things, there are two different types of AMD64 chips. The "normal" AMD64 chips (rated the same as XPs) are socket 754 while the "FX" AMD64 chips are socket 940. The FX chips are kinda like the Xeon of AMDs, I guess. They're more expensive, as are the boards. Probably overkill for a desktop scenario too.