I have this problem with my gfx. When crossfired amd apu with 6670 at battlefield 3 I get more FPS than stand-alone 6670, but the overall screen movement on the apu+6670 is very choppy, almost unplayable. Meanwhile why I get less FPS on aline 6670, I can easily look around and not notice (almost) any stutterness. What's the problem?
the performance of the crossfire will vary not only in fps but also in general smoothness. Some games will not be playable with crossfire due to the performance difference between the integrated and discrete gpus as well as the driver and optimizations in game.
If you want better performance, returning the 6670 and getting a more powerful graphics card and not using crossfire will still be perfectly fine( such as getting a 7850). You don't really need an i5 to play games but it does give the best performance if you want to spend the money.