For a system using a single Radeon HD 7950 graphics card AMD specifies a minimum of a 500 Watt or greater power supply. The power supply should also have a maximum combined +12 Volt continuous current rating of 30 Amps or greater and have at least two 6-pin PCI Express supplementary power connectors.
There are non-reference design Radeon HD 7950 graphics cards on the market that require two 8-pin PCI Express supplementary power connectors.
There are also some cards with a boost BIOS that will require you to increase the PSU's maximum combined +12 Volt continuous current rating to 37 Amps or greater because of the increased power draw due to the higher clock speeds.