I wouldn't * risk it, that PSU only has 504W on the +12v rail, which is 42A, which is very low, in fact, a quality 550W psu has more amps(or about the same), it clearly has a fake wattage, also for card requirements check it here : http://www.realhardtechx.com/index_archivos/Page362.htm
That psu is made by AcBel Polytech, that doesn't seem good either.
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 system power supply that has a maximum combined +12 Volt continuous current rating of 53 Amps or greater and that has at least four 6-pin PCI Express supplementary power connectors or two 6-pin and two 8-pin PCI Express supplementary power connectors depending on brand and model of the graphics card.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Combined Continuous Power/Current Available on the +12V Rail(s) rated @ 45°C - 50°C ambient temperature, is the most important factor.
Overclocking of the CPU and/or GPU(s) will require an additional increase to the maximum 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 being attempted.
The GIGABYTE Superb 720 (GE-P610A-C2), with its maximum combined +12 Volt continuous current rating of 42 Amps and with two (6+2)-pin PCI Express supplementary power connectors, is grossly insufficient to power your system configuration with two GeForce GTX 680 graphics cards in 2-way SLI mode.