AMD cpus use a rating system.
i.e. a 2600XP is supposed to have a comparable performance to a P4 2.6Ghz.
If buying completely new system go for Athlon 64 3500 or 3800, if you don't mind spending the big bucks go for the FX53.
All three are streets ahead of a similarily priced P4. The FX53 is THE cpu for gaming, but comes at a cost. Check
http://www.pricewatch.comAccording to what I've read here nVidia seems to work better so go for a 6800. Also works better if you decide to run Windows XP64.
My system (check sig) with vsynch on (85hz) 1280x1024x32 gets steady 85fps. vsynch off seen it as high as 540+.
The socket 939 Athlon 64 3500,3800 and FX53 all use dual channel memory setups, so you will need 2 x DDR400 memory sticks to make the most of it. Tip here get Corsair TwinX memory, guarenteed to work at advertised speeds together. Also make sure they a LL modules, Athlons work best with Low Latency memory ( CAS 2).
Last thing- if you get socket 939 Athlons, ensure board is socket 939 also.