32 vs. 64 bit question.

So I just finished building a new system with 4 gigs of RAM and n00b me, I never knew about the 4 gig memory cap in a 32 bit operating system. Only 3.25 are showing up. So to get the full 4 gigs to show up in Windows without dealing with the /PAE switch, I understand I need a 64 bit version of Windows. My question is, to run a 64 bit version of Windows, do I need a 64 bit processor too, or can I install it on a regular dual core Athlon FX?

I wouldn't recommend going with XP Pro x64... if you really want 64-bit, you'll be better off with Vista. The driver support is starting to come together nicely for Vista x64... but there is still a big lack of drivers for XP x64.