For a system using two Radeon HD 7970 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 750 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 59 Amps or greater and have at least two 6-pin and two 8-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 is the most important factor.
The Corsair Professional Series Gold AX850 (CMPSU-850AX), with its combined +12 Volt continuous current rating of 70 Amps and with four (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with two Radeon HD 7970 in 2-way CrossFire mode.