New demanding games won't allow you to get near 120 FPS, but indie and older games will at times, with a single 780 ti. You may also learn that it is impossible to get enough GPU power to get 120 FPS in a lot of games, indie, older or new. This is because some games are CPU bound, even with the fasted CPU's there are.
You may find 780 SLI would be a better compromise, for that 1440p 120hz monitor.