120 FPS is not what you get on a minimum, even with SLI, rather at max in select games, with hammering titles like Crysis 3, you're not going anywhere near 120, not even 100. Look up some benchmarks. And up from 50 I'm certain its mostly likely a matter of 'oh see I got 75 FPS in benchmarks/ gameplay'.
IMO, 30+ is a must, 40+ is decent, 50+ is comfortable, 60+ is excellent, higher is always better, but is it necessary? Well, if we take it like this, that we do see a difference in 30 and 60 FPS, but rarely in 60 and 120 FPS, then IMO, no, it's not necessary, I'll be happy with 40+ FPS on ultra, but that's just me. You can put 980 three way SLI for a single FHD, no one stops you, and get 100 FPS, but is it worth it? Nope.
I'd go for atleast a 1440p monitor for SLI 970 setup, otherwise I'm more interested in frames.