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 27 Amps or greater and have at least one 6-pin PCI Express supplementary power connector.
For a system using two Radeon HD 6850 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 600 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 38 Amps or greater and have at least two 6-pin PCI Express supplementary power connectors.
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 GIGABYTE PoweRock 500W (GE-N500A-C2), with its combined +12 Volt continuous current rating of 36 Amps and with two (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with a single Radeon HD 6850 and handle some overclocking of the CPU.
The GIGABYTE PoweRock 500W (GE-N500A-C2) is based on CWT's DSA Series platform.
The Corsair Builder Series CMPSU-430CXV2 and CMPSU-500CXV2, recommended by @amuffin above, are based on CWT's DSA II Series platform.