Here is Guru3D's power supply recommendation on the Radeon HD 6800 series:
•Radeon HD 6850 - On your average system the card requires you to have a 450 Watt power supply unit.
•Radeon HD 6870 - On your average system the card requires you to have a 500 Watt power supply unit.
If you are going to overclock CPU or GPU, then we do recommend that you purchase something with some more stamina.