For a system using two GeForce GTX 680 graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 750 Watt or greater power supply that has a combined +12 Volt continuous current rating of 53 Amps or greater and that has at least four 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important factor.
Overclocking of the CPU and/or GPU(s) will require an additional increase to the combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock you are trying to achieve.
The Cooler Master GX Series GX-750W (RS-750-ACAA-D3 / RS-750-ACAA-E3), with its combined +12 Volt continuous current rating of 60 Amps and with four (6+2)-pin PCI Express supplementary power connectors, is sufficient to power your system configuration with two GeForce GTX 680 in 2-way SLI mode.
If you have the now discontinued RS-750-ACAA-E3 model, made by Seventeam, there is a problem with high electrical noise on the +3.3 V and +5VSB rails that will exceed the maximum allowed by ATX12V specs when 600 Watts or more is drawn from that power supply unit.