For a system using a single reference clocked 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 24 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 Combined Continuous Power/Current Available on the +12V Rail(s) rated @ 45°C-50°C ambient temperature, is the most critical factor.
The Antec EarthWatts Series 500W (EA-500D), with its combined +12 Volt continuous current rating of 34 Amps and with two 6-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with a single Radeon HD 7850.