For a system using a single Radeon HD 6850 graphics card AMD specifies a minimum of a 500 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 25 Amps or greater and have at least one 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT a crucial factor in power supply selection!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important.
I checked the power supply I've selected and I have 35A so I'm good. Along with the 6-pin connector.