For a system using two reference clocked GeForce GTX 670 graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 700 Watt or greater system power supply that has a maximum combined +12 Volt continuous current rating of 45 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 Combined Continuous Power/Current Available on the +12V Rail(s) rated at 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.
Since the Gigabyte GV-N670OC-2GD is a non-reference design factory overclocked graphics card it will consume 18 Watts more during gaming and 80 Watts more during FurMark stress testing for each graphics card. If you want your system to be able to survive GPU stress testing then allow at least a maximum combined +12 Volt continuous current rating of 55 Amps or greater when using two of those graphics cards in 2-way SLI mode..
The Corsair HX Series HX750 Power Supply (SKU# 75-001218 / CP-9020031-NA), with its maximum combined +12 Volt continuous current rating of 62 Amps and with four (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with two GeForce GTX 670 graphics cards in 2-way SLI mode.