For a system using a single Radeon HD 7850 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 26 Amps or greater and have at least one 6-pin PCI Express supplementary power connector.
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 factor.
The OCZ Fatal1ty Professional Series 550W (OCZ550FTY), with its combined +12 Volt continuous current rating of 37.5 Amps and with one 6-pin and one (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with a single Radeon HD 7850.