You can get a 780 for 480$ now, which I would strongly recommend you do that rather than SLI'ing 'mid-end' cards.
760 SLI really isn't impressive, I've never used them like that, a buddy of mine runs SLI 660 ti's in the same tier of GPU, and my single 780 gets about 10~ more fps
Thats upto you though, the performance is similar, fair warning it will make your room hot, and your pc will run a lot harder in SLI