What I'm saying is, games generally have a fixed amount of CPU needs. If a game can only run up to 40fps on a Core i5, it will only ever reach a maximum of 40fps, regardless of what monitor or GPU you have. However, it's very possible for it to run at less than that, if you increase your resolution or raise graphical settings enough.
Realistically, your i5 is plenty of CPU for 99.9% of games out, and you will not be limited to the hypothetical "40fps" scenario I made up above to illustrate a point.
Your GTX 960 should be able to run the vast majority of games at very high settings at 1366x768. If you move up to a 1920x1080 monitor, you may need to drop your details down to medium or high in some games, so that your GTX 960 can still maintain the same framerates, but I think it's still plenty of GPU power to run games nicely at 1080P.
If I were you, I would get a nice 1080P screen, somewhere between 23" and 27".