I cannot find any specs on your power supply. That leads me to conclude that it is a cheapy.
Figure 10 - 12 amps (at 12 volts) for the 6850 and 10 amps for the CPU. Figure another 3 amps for HD, optical, and fans. That's 23 - 25 amps just at 12 volts. That doesn't include the 5 volts for the digital logic or 3.3 volts for the memory.
The absolute bare minimum PSU I would use is the now discontinued Corsair 400CX with its 30 amp 12 volt rail.
I'd probably use a good 550 watt PSU (Seasonic, Corsair, XFX, or Antec), certainly nothing less than a good 500 watt PSU.