True but if the OP purchases a 7850 he wouldn't be using it to its full potential which sort off defeats the point of making that kind of purchase.
This is true, but the OP could always OC the q6600 w/the 7850 and get more FPS out of his system (assuming he hasn't already OC'd the q6600). My point is that if the OP wanted to buy the 560 Ti or 7850, he could use it (not to it's full potential) and then transfer it to a newer system later down the road. Yes he could save some $ now and just spend ~$150 on a 6850 or something like that and use the $100 later to upgrade his system. Now the 6850 will be less of an upgrade to the newer system, but that would be up to the OP. I know Tom's did an article on minimum CPU's needed to get the most out of some GPU's, but I can't find it at the moment.
I probably don't have enough cash for a full system upgrade so I was just trying to do piece by piece until its all upgraded.
I would buy the best GPU you can afford and then OC the CPU to get maximum FPS. You will probably need to upgrade your CPU HSF if you OC quite a bit, but you should be able to OC to 2.8-3.0 without too much troubles. Of coarse you will need to have a quality PSU to OC much.