For a system using two AMD Reference Design Radeon HD 7970 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 750 Watt or greater system power supply. The power supply should also have a maximum combined +12 Volt continuous current rating of 58 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!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most critical 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 you're planning on overclocking I would recommend a Seasonic 850 Watt or 860 Watt PSU to ensure that there is adequate +12 Volt continuous current available when needed.