For a system using two Radeon HD 7950 graphics cards in 2-way CrossFire mode AMD specifies a minimum of a 750 Watt or greater system power supply. The power supply should also have a maximum combined +12 Volt continuous current rating of 45 Amps or greater and have at least four 6-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.
The Rosewill CAPSTONE Series 750W 80 PLUS GOLD Certified (CAPSTONE-750) Power Supply, with its maximum combined +12 Volt continuous current rating of 62 Amps and with four (6+2)-pin PCI Express supplementary power connectors, is way more than sufficient to power your system configuration with two Radeon HD 7950 graphics cards in 2-way CrossFireX mode. There is more than enough spare capacity on the +12V rail to handle any overclocking of the GPUs you may want to attempt.