I also have a Q6600 .
Its a ok processor, but I can notice it becomes slightly outdated. I wouldn't buy a new one, but it's something you need to keep in mind when you buy a GFX card. The question is : are you planning to keep updating your system or will you buy a new system from scratch in the future ?
If it's the latter one, there is no reason what so ever to buy a top notch card, you'll have to look on the cards performance in relation to your processor and go for the best preformance/ price ratio for the card your system can handle.
I can tell you that it feels like my system with a GTX275 seems to start hitting the roof of what I graphically can squeeze out of games. For me, I don't think mounting any better card would do me any good, as the processor seems to be the bottleneck on my system