I have a similar question, was going to create a new thread but I'll post it here. I have a Core 2 Quad Q8200 that is 2.3 GHz.
Would it be worth it for me to get a new case/PSU so I can add a Radeon 6850 or a 6870? The upgrade would cost me $270.
Depends what you're upgrading from. But the "CPU bottleneck" issue won't be too bad. Have you tried OCing? If you can get it up to 3ghz or more it'll help.