GeForce GTX 780ti - requires 42A on +12V rail and a 600W PSU minimum
Power Consumption GeForce GTX 780 Ti 2-way SLI
1.System in IDLE = 129W
2.System Wattage with GPU in FULL Stress = 617W
3.Difference (GPU load) = 488W
4.Add average IDLE wattage ~20W
5.Subjective obtained GPU power consumption = ~ 508 Watts
GeForce GTX 780 Ti 2-way SLI - On your average system the cards require you to have a 800 Watt power supply unit as minimum.
So yes you are fine to go.