For a system using a single Radeon HD 6990 together with a single Radeon HD 6970 graphics cards AMD specifies a minimum of a 950 Watt or greater system power supply. The power supply should also have a maximum combined +12 Volt continuous current rating of 56 Amps or greater and have at least one 6-pin and three 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...