You can purchse a decent gaming system nowadays for around 500-700 bucks, if you're interested 🙂
I have great faith in the Athlon XP chipset - I use a 2100+ right now, and I can run anything pretty darn well. On my current box, I'm running an MSI motherboard, which has given me minimal problems. I also really like (I'm gonna get blasted for this) Gigabyte motherboards. The pretty colors entrance me.
As for a GPU, I would still stick with a G4 Titanium - the tested and true of the Geforce line. While you could spend the money on a 9600 or 9800 Radeon, the deal is, the next generation of procs and GPUs are going to be coming out soon, so you'll want to upgrade anyways. A G4 TI4800 would hold up well, and you won't feel bad about replacing it in a year or two!
Good luck with your computer, and make sure to listen to everybody's opinions! While I know my way around a box, there are a lot of knowledgeable people on these forums!