I agree with Skyscraper. 2GB RAM is plenty.
My XP legacy box only has 1GB of RAM. It almost never gets maxed out. One of these days, I'll upgrade it to 2GB just to be obsessive-compulsive.
If you're an NVIDIA type of person, the 7950GT is the fastest AGP card you can get. However, I remember reading of weird failures with those cards. That is why I chose a 7900GS AGP card for my system. It seems like the fastest, most stable NVIDIA AGP card you can get. Really though, your Athlon XP won't utilize all of that power.
If you want to dual-boot XP & Win98 and performance is your biggest concern, then get a 6800 Ultra. If you want a VGA as well as DVI connector, get the 6800GT card. If you're concerned about compatibility, then get a 5950 Ultra or 5900 Ultra.
If you like Radeon cards, the 4670 is the absolute fastest AGP card you can get. The 3850 is also a good choice and you can run Omega drivers with it. Both of them blow away the NVIDIA 7950GT. Your Athlon XP 2200+ definitely won't take advantage of them though.
My recommendation: NVIDIA GeForce FX 5950 Ultra or Radeon 9800XT. Both of those are good matches for your Athlon XP 2200+ CPU. GeForce FX 5900 Ultra and Radeon 9800 Pro are also good. They're slightly slower, but run cooler and consume less power.