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 connector.
The BFG 450-Watt power supply should have a combined +12 Volt continuous current rating of 25 Amps and have one 6-pin PCI Express supplementary power connector so it just meets the minimum power supply requirements.
Just don't plan on doing any overclocking though because that will increase the power draw beyond what the power supply is able to deliver.