The eXtreme Power Supply Calculator results in almost 400 watts as the recommendation. Personally, for this system, I would get a 500-550 watt PS as a minimum. XFX Core Edition 550 watt Pro (Seasonic) is a good one. Another is the Antec HCG-520 or HCG-620 (Seasonic). PC Power & Cooling Silencer Mk3S500 500 watt or Mk3S600 600 watt (Seasonic) are other choices. If you are planning to crossfire with another 6850, get the 600 watt PS. If single 6850 only, a good quality 500 watt should do.