Before you buy let me ask you a few questions. What resolution do you game at, what are your most commonly played games, and what power supply is in your rig right now?
The reason I ask is because the q6600 you are currently using is not a bad processor, it will run most games just fine. The q6600 is pretty close to the amd x4 645
AnandTech | Bench - CPU
and that processor was compared to several high end modern cpu's with a 7970(670/680 like speed) to check for bottlenecking.
Radeon HD 7970 CPU scaling performance review - Introduction
As long as your playing at a higher res than 1600x1200 you should be fine with the cpu you have for a little while longer. I would suggest you invest in a modern mid tier gpu from your favorite manufacturer.