Which Core 2 Quad do you have? There were a number of models that varied in clockspeed from 2.33GHz to around 3GHz. Kentsfield C2Qs (Q6XXX) are also slower clock for clock compared to Yorkfield C2Qs (Q8XXX or Q9XXX). If you have an older C2Q you are more likely to hit a CPU bottleneck if you get a higher end card, though that can be alleviated somewhat through overclocking, a Kentsfield C2Q is still decent at gaming if you can get it up to around 3GHz.