What about upgrading the CPU in the current motherboard. Shouldn't be to expensive huh!? Question is... What CPU then?
I think you should overclock the Q6600 and get a 5850. Take the savings from 5870/GTX470 and save it towards your next upgrade, say in 18 months. I wouldn't bother upgrading your Socket 775 CPU since you have P965 chipset. On that chipset, you are better off overclocking Q6600 with 266FSB, than trying to overclock a 45nm Penryn which is not successful beyond 400-415FSB even on P35 chipset (excluding say Abit IP35 boards. I even doubt that 45nm is even supported on your board).
An overclocked Q6600 @ 3.2ghz isn't going to be much slower than say a Q9550 @ 3.4ghz (which is likely the very maximum your board will do, even if it supports it). The real difference in CPU speed is with Core i7 processors, but in your case that means changing the motherboard and RAM. I still think bang for the buck, 5850 purchased today + $100 savings towards a new graphics upgrade in 2 years is better than getting a 5870 right now. You can overclock the 5850 if anything, but 5870 has hardly any more overclocking headroom. This is why 5850 is a better deal overall, especially since you don't have the CPU speed to take full advantage of the GTX480 or say 5870.
Put it this way, even an Athlon II X3 440 with 5870 in CF cannot consistently outperform a Core i7 920 stock with a single 5850 in a variety of games at 1920x1080:
http://www.tomshardware.co.uk/athlon-ii-x3-440-gaming-performance,review-31906-9.html
Now a stock Q6600 is probably 50% or more slower than a Core i7 920. This is why I do not recommend you spending 50% more $ on a 5870 with such a processor for 1920x1080 resolution unless you plan on using 8AA to shift the bottleneck towards the GPU.
For example, in Starcraft 2, Q6600 stock combined with a 5870 cannot even manage 30 fps average at a lowly 1680x1050 resolution with 0AA/0AF:
http://www.pcgameshardware.com/aid,...CPU-benchmarks-x-Core-i5/i7-leading/Practice/
In Borderlands, Q6600 stock cannot manage > 30 fps minimum frames and averages at 60 fps at 1600x1200 with 0AA:
http://www.techreport.com/articles.x/18799/6
I am not saying that 5870 isn't a great card -- it is. However, in your situation, the 25%-30% performance advantage it enjoys over 5850, when combined with a high end Core i7 or Phenom II processor, will be all but eroded due to the lowly 2.4ghz speed of your processor given your resolution.