System Power Supply Requirements for two NVIDIA Reference Design GeForce GTX 980 Ti graphics cards in 2-way SLI mode
• NVIDIA specifies a minimum of a 850 Watt or greater system power supply. (Minimum system power requirement based on a PC configured with an Intel Core i7 3.2GHz 130 Watt TDP processor.)
• the system power supply must also have a maximum combined +12 Volt continuous current rating of 60 Amps or greater
• the system power supply should also have at least two 75-Watt 6-pin and two 150-Watt 8-pin PCI Express supplementary power connectors. There are some non-reference design cards that require two 150-Watt 8-pin PCI Express supplementary power connectors (E.g. EVGA GeForce GTX 980 Ti Classified ACX 2.0+, GIGABYTE GV-N98TG1 GAMING-6GD, MSI GTX 980TI GAMING 6G) per card.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) is the most critical factor.
Overclocking of the CPU and/or GPU(s) may 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.
Your CPU has a TDP of 140W so you'll need a PSU that has a higher +12 Volt continuous current rating than what's mentioned above. If you overclock the Intel i7-5960X alone the extra +12V current required can reach 10 Amps or more so you would need to add 10 Amps to the above 60 Amps.
An 850W PSU is the minimum. 750W is insufficient to handle overclocking of the CPU and GPUs.
Corsair RM850i (SKU# CP-9020083)
• OEM: CWT
• maximum combined +12 Volt continuous current rating: 70.8 Amps <===== Sufficient
• six 150-Watt (6+2)-pin PCI Express supplementary power connectors <===== Sufficient
• Full Modular Output Cables: Yes
• Official Intel Haswell Compliance: Yes
• 80 PLUS GOLD Efficiency Certification
• 7 Year Limited Warranty