High end C2Qs are a bit too expensive these days.
And yeah, you're CPU is a limiting factor. The highest you can go with that, would be a HD 6850 or a GTX 460, and too, with slight bottleneck.
You'll need to either overclock your current CPU, or change the motherboard and buy a new CPU. That'll cost more, but it will be more, future proof...
Since you're willing to spend $600, I'd suggest you get a better CPU and motherboard. But, if that is the case, ten you'll need to change your RAM too.
Cheaper way - overclock your CPU, buy a Radeon HD 6870 or HD 6850
Future proof - Buy better motherboard, CPU and RAM. http://www.tomshardware.com/reviews/core-i5-overclock-performance-gaming,3097.html
For almost $600, you can buy yourself a good rig too.