The Q9450 is around $480 here in Aus, which would be your whole budget. And considering an i7 920 is about the same, imo it is not worth it. You would need to spend alot more on an i7 build, but at least that would see you a significant jump in performance. Going to a Q9450 would not see a significant jump, at least in games, and your E6850 can probably overclock past it anyway.
Do you have an SLI motherboard?? If so you could invest in another gtx 260 which would then eat just about any game out there.
Try overclocking, then if you still want to consider a cpu upgrade, its only worth it to go to an i7 rig.