The CPU does not affect the image the card puts out, but it does affect how quickly said card gets information and thus how fast it preforms. Anyway, a Q8200 is a bit weak for a 6870 so if you can overclock your CPU then I would suggest you do so. If not, then save some money and go with a cheaper 6850, 5850 or GTX 460 1GB. Just make sure your PSU is sufficient.
Yes, mega is right. If you can get that to 3.0Ghz, you should be fine with a hd6870, 2.8Ghz hd6850, 5850, and GTX 460 1gb. You can OC those pretty far with a small voltage bump.
How can I overclock my CPU while I have an Intel DG35EC Mobo...
Is it an OEM machine? Basically a machine bought from a company e.g. Dell, Gateway, HP, etc. If so, you can't OC it. If not, watch a video on youtube about it. It's pretty easy. You will basically put up your base clock or your front side bus. You will put your RAM speed back, and then put soem other settings up which I am not going to go into. Just watch a video about it.
One thing is that you take into account some physics processing technology such as Havoc still let the processor do all the physics.Such as gravity, breaking of wall features etc.