i would go for the 7870, only because i don't trust the power consumption of nvidia cards anymore.
Except that, your cpu will be the bottleneck no matter what card you will buy .
I had i7 920 overclocked at 4ghz with my ati 5870 and when i upgraded to i5 2500k because of motherboard failure i had a huge increase in fps avg and minimum.
Maybe you should give these 300bucks for z68 motherboard and i5? (With z68 chipset motherboard you will use the same ram you have and you will have the ability to insert later on even the ivy bridge cpus).
I guarantee you, i run, for example crysis2 with dx11 patch and high res textures and latest patch at 1920x1200 all details! full with no lag or low fps. (above 30). This never happened with i7 920 at 4ghz. And you don't have to buy i5 2500k (although with a good cooling system you can go up to 4.5ghz..). even the new i5 35xx are awesome and nicely priced