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 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 Antec High Current Gamer Series 900 Watt (HCG-900), with its maximum combined +12 Volt continuous current rating of 70.8 Amps and with four (6+2)-pin PCI Express supplementary power connectors, is way more than sufficient to power your system configuration with a single Radeon HD 6990 together with a single Radeon HD 6970 graphics cards.