For starters the 500 series cards are the same cards as the 400 series pretty much. They just got rid of some of the things they messed up on the 400s. Added better cooling, and made better drivers. I would go for the i7 personally. Though joel is correct in saying the gpu is normally the bottleneck for most gamers not the cpu in this case 2 years from now when you want to upgrade your graphics card to the newer models (most people do every 2-3 years) you will probably be bottlenecked by the i5.
Also I say this because currently the i7 can be found for aproximatly 50$ more than the i5 if you know where to shop so to me that extra 50 bones is worth the extreme increase in performance. Just make sure you have proper cooling for your system.