Also remember that in some games, like Crysis 3, you will not be able to get 120 FPS at max settings, even with something crazy like 3x780s, because your CPU will limit you even if you have like a 5 GHz 3960X.
I also recommend 770 SLI for 120 FPS gaming. You'll also probably want a 4.5 GHz+ 3570k/4670k
LOL, that's true. I have a tri-SLI 780 and an I7-4770k @4.8 GHz and I can't barely reach 120 FPS in Crysis 3.