If you want the fastest gaming system, you should decide by video card first.
Since you don't want SLI or Crossfire, that leaves your choices as the Radeon X1900XTX and Geforce 7900GTX (or Geforce 7800GTX 512) as your two fastest choices, though most will go with the X1900XTX.
Next you want AMD, they are far superior for games. Since you just want dual core, your choices are....
Athlon X2 3800+ to.....I don't know what they go up to, 4600+ maybe?
As well as some of the Opterons, like the Opteron 165.
As well as the Athlon FX-60.
I'd say a 4600+ or 4400+ would be a better use of your budget, or even a 4200+.
Also, get 2 1GB sticks of PC3200 ram, since it can't be a high end gaming PC without 2GB of ram.