I use an AMD 2500+ processor and it runs UT2K4 just fine. although, i think the processor is behind video card and RAM when it comes to running a game like Unreal Tournament.
I think the biggest factor in running this game would be a good video card. If you have a low-end video card, you're not going to get very good results. The better your video card, the better the game will be.
its ok, if you have the money go for a 5950 ultra, or if you have even more for the 6800 ultra or the x800. Im gonna have the 6800 very soon, the benchmarks are awesome on it.
Wait a couple of years! In a few years time you'll probably be able to buy a secondhand 64bit CPU base system for about £100. Stick with the older games for now!
I'll be hated for saying this but paying so much for a top end PC just to play games is a complete waste, especially seeing that PC prices are very unstable.