If you play at a high resolution and enable all the eye candy then the video ard will matter more and the CPU less. Even if the card is unable to work at 100%, I'm sure it will still be a good gaming machine.
Eroc, is your CPU a Manchester or a Brisbane? That is, socket 939 or AM2? If it's socket 939 you'll have a very hard time upgrading the CPU, unless you buy a new MB too. If it's AM2 it should be easy.
AM2, i built this machine last year, i was debating building a new one with a q6600 and everything, but i figure this machine can hold me over with a basic GPU upgrade; the 7900gs i have in it is really not enough anymore. besides with nehalem around the corner i figure i should just wait