you need at least a 500 ( more like 600)
Nope cause between the CPU and the 6850 GPU the PC will be pulling under 200 Watts that leaves an overhead of 200watts to run the other parts with.
i3 CPU = 65watts max
6850 = 125watts max
so that leaves 200wats to run the Mobo, HDD, Ram and Optical drive witch 200watts is plenty more than enough.