You should be able to overclock the C2Q to 3.2GHz. Watch out for the RAM speeds though - if you can find DDR2-8500 at a reasonable price, buy some, as it will allow you to push the CPU clock higher before introducing a divider on the front-side bus, or FSB.
As for the graphics card, a GTX960 will work well, but even at 3.2GHz (the G0 stepping is regarded as the best overclocker of the Q6600 line; I never managed to push mine stable far past that frequency), you are looking at the CPU being the bottleneck
in CPU-intensive games. This rig's CPU would be the bottleneck in Battlefield 3, for example. Whether or not E|D will suffer from it remains to be seen. I will be able to tell you by the end of the day - I have an almost identical rig sitting on the floor next to me (my wife's old PC with a GTX670 2GB) which I'm going to turn into a secondary machine for multiboxing and in-home streaming.
Questions to consider:
- What kind of resolution do you expect to be playing at? What kind of compromises on graphics are you prepared to accept?
- How much time are you willing to spend tracking down replacement parts if one fails due to age?
- Are you comfortable overclocking? Do you have an aftermarket CPU cooler? (You'll need one!)