For a system using two Gigabyte Radeon HD 7950 graphics cards in 2-way CrossFire mode a minimum of a 750 Watt or greater system power supply is recommended. The power supply should also have a maximum combined +12 Volt continuous current rating of 52 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.
The Corsair CX750M (SKU # 75-002019 / CP-9020061) power supply unit, 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 Radeon HD 7950 graphics cards in 2-way CrossFireX mode.