I've done some searching around, but haven't really seen a good answer to the following question:
How much GPU horsepower do I need to run games on a 1920x1080, 120 hz monitor? Is any single GPU going to be sufficient, or will I need multiple GPUs? Nvidia vs. AMD? I'm thinking games like Skyrim, Far Cry 3, Crysis 1-3, Mass Effect, etc.
For 120 FPS (though you can play at 100hz and 100 FPS and still have pretty optimal conditions) takes both a fast GPU and CPU. In some of the most demanding games, your CPU will limit you, so don't skimp there. I'd say 2 770's would be optimal for 120 FPS. Perhaps two 760's would work as well.
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.