its a little more complicated than that. A lot of it depends on where the game does its physics work. If it is offloaded onto the CPU, then a weak CPU can hold you down. If the GPU is the physics engine, then it will not matter as much.
the recommendation though, doesnt change. The AMD/AMD will give you more FPS than the Intel/AMD
Still, in the grand scheme of things, the 6850 is still an insanely good GPU. Basically, I'm looking at it in the sense that the 6850 is less worse than the 6870 than the Phenom II 955 is to the 2400. Therefore, I think that the i5 combo would prove to be faster than the Phenom combo.The cpu may determine fps but a quad core phenom will in now way create bad frame rates in the sense that they are unplayable, a worse GPU might. Sandy bridge may have the faster over all fps but a cpu can maintain above 60 fps, which i believe the phenom can, i really dont see the point in getting a better cpu and worse gpu, the phenom wont struggle in a way that would make any game unplayable but the HD6870 might
Better the have the better GPU imo. We could also start the debate on weather they phenom whould be a good investment since bulldozer is around the corner. But that's a whole other can of beans.