http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
I don’t recommend upgrading your graphics card unless the replacement card is at least three tiers higher. Otherwise, the upgrade is somewhat parallel, and you may not even notice any worthwhile difference in performance.
So..... to jump 3 tiers above e 750 , you'd need at least a GTX 660 Ti, GTX 760, HD 7870, R9 270, 270X
4 tiers u p.... GTX 580, GTX 670, HD 5970, 7870 LE (XT), 7950, 280, 285
5 tiers up .... GTX 590, 680, 770, HD 6990, 7970 GHz Ed, R9 280X
6 tiers up ..... GTX 780, 970, Titan R9 290, 290X
Keep in mind that is if using at stock speeds "outta the box" ..... if OC'ing you get more of an OC bump outta the nvidia 7xx / 9xx cards...