Ummm you don't need much of a machine at all - it's basically going to be a router. Anything that has two decent network cards in it will do if it's not going to be running anything else.

But to your questions

RAM 32bit operating systems don't have enough addresses to see more that 3.5 gigs of RAM. So you can put all the ram the motherboard can use it, but XP won't be able to use it.

Yes your 32bit operating system will run fine on an AMD 64bit chip.

AMDs make great machines, don't worry.

Again, why a 500 gig drive for a router? Are you intending this box do anything else?

Power supply - ALWAYS buy a good powersupply regardless of your abilities. The power supply must be able to supply clean power at certain specifications all the time. This is more critical than your choice of motherboard!

Finally, so few pci/pci-e slots... why is this a concern? What else is this box going to do?

You need one pci slot for another network card - I would suggest you get a gigabit card to match the speed of the onboard network (lan).

If you post what you intend to use the box for, I can be a bit more specific. Personally, if it was just to use the routing software for your guests, I would shove it on an older 2ghz (or there abouts) box with two new network cards in it.