I am thinking of 144hzYou didn't mention what kind of performance you're looking for and in what games.
Assuming at least 1080p 60hz gaming with games that are primarily graphics oriented then that upgrade will vastly improve things. A 750 Ti hardly even counts as an entry level card anymore.
You'll know if you need an i7. What'll happen in CPU demanding games is your average framerate might seem alright but your framerate will drop pretty low, and often. This will be especially true in highly populated multiplayer games. Having a good average fps and having a stable and good average fps are too different things.