For a system using two Radeon HD 6950 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 700 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 50 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!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important.
The OCZ Fatal1ty 750W Series (OCZ750FTY), with its combined +12 Volt continuous current rating of 54 Amps and with four (6+2)-pin PCI Express supplementary power connectors, is sufficient to power your system configuration with two Radeon HD 6950 in 2-way CrossFireX mode.