Two 760's in SLI will outperform a single 780ti by a noticable difference if the game is properly optimised for SLI. And no matter your setup, 650w would not cut it for SLI 780ti. If you want to be in the safe zone get a 80+ gold certified 800-850w. This will leave lot's of room for overclocking or anything else that you might change in your system.
EVGA Supernova G2 850w has an excellent score on Jonny guru. 9.9/10. http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story6&reid=377