According to Computer Gaming World, for what's worth anyways, they did an Ultimate Gaming Machine test. 6 of the 12 machine were running AMD FX51 and they say it is the processor to have for gaming. Personally I have a 3.06 P4 and it is very fast, and alot cheaper!
There is an article (CPU Buyer's Guide) on THG you should check out.
I wouldn't recommend getting either of those due to price performance ratio. You'll spend 750 just for the Fx chip and 1000 for the p4.. and they aren't really that much better. In fact there really isn't enough games out for pc to want to spend that much.
9800Pro with a 2 ghz processor and above 512 ddr memory is more than good enough to run all the games and even Doom3 on full settings.
But as the other poster said you should take a look at the cpu buyer guide it will help you out.
p4 2.8 533fsb
1gb rdram pc 800
120gb seagate s-ata