I have a massive performance drop when I enable SLI (maximize 3d performance) in Nvidia control panel.
After I enable it and run 3dMark (Firestrike) it drops to 10 frames vs. 30 frames without SLI enabled. (doing just the fist test)
Definitely not the PSU. If you had insufficient power to a component, the PC would BSoD or just shut down. Almost definitely your CPU is bottlenecking your system. If you are limited in funds I'd recommend taking one of the 760's back and getting a new mobo + latest gen i3-4xxx or an I5-4xxx if you can muster a little more cash.