Your existing MoBo is SLI capable. A 750 watter will be fine if you don't overlcock but with two GPUs and a CPU OC'd I'd want a quality 850 watter ?
Is your 680 a 2 slot or 3 slottter ? If the latter are the PCiE slots too close to fit another ?
If ya wanna go new, I'd opt for a MoBo with wide spacing between the PCI-E slots (cooling) and a 3570k. I have always been partial to the Asus WS series for this reason.