In terms of the right speed CPU to pair up with your GPU, if you have a high-end GPU (X800 or 6800GT class) then the faster you go the better off you are. Mid-range GPU owners will find that anything the speed of an Athlon 64 3000+ (Socket-939) will offer the best bang for your buck, with diminishing returns after that. If you happen to have an older Radeon 9600/9700/9800 based card, then even an Athlon 64 2800+ will be overkill for your GPU. If you are stuck with one of those older but still well-performing GPUs, don't bother upgrading your CPU unless it's something slower than a 2.4GHz Pentium 4 - you'd be much better served by waiting and upgrading to dual core later on.

The impact of the CPU on gaming performance is in a transitional stage right now. As more games use Half Life 2 style physics we will see similar impacts with regards to CPU performance, but at this point there's a great deal of work being done on multithreading game engines for the next generation of games. So while the games coming in the immediate future may behave similarly to Doom 3 and Half Life 2, it's the games that follow that will truly be interesting.