For the most part the i5 4670k will be better. If you were to get, for example, an R9 280x you would not notice any difference between the CPUs. With your GPU, you might. Since anything over 60FPS you won't actually see, and both CPUs can achieve that.
The i5 4670k will be better for single-threaded (older) games, and the FX and i5 are similar in better threaded games (2-4+). And then with the FX you have a few cores idle available for other multitasking. But overall the i5 4670k will perform better, but the disparity is difficult to claim. Personally, I went with an FX 8320 as I'm not going to pay so much more just for a minimal FPS gain, you can get more out of a good GPU. You're correct, some games are optimized for either brand.