You probably won't be utilizing the full capabilities of your monitor in most games, but the great thing I've found about 120hz+ monitors is that you can tweak the in-game settings and get optimal performance based on your preferences. I've always preferred smooth gameplay over beautiful visuals, so if turning down a few settings nets me a solid 80-120 FPS in a game I'm more than willing to do that as opposed to maxing out a game and getting a consistent 50-60 FPS. With a 60hz monitor, the latter is the best solution, but at 120hz I much prefer aiming for the higher framerates that my monitor can output. Personal preference of course.
But yeah, you're likely going to need to go SLI in the future if you want to get the most out of your new monitor, especially in the upcoming BF4 which should be fairly demanding.