A 6850, at best, needs about 10 amps from the 12 volt rail. Figure another 10 amps for the CPU. That's 240 watts. Figure about 60 watts for the drives and 80 watts for the motherboard and the RAM. Fans - another 10 watts.
A 400 watt PSU would do, but you have no reserve. A 500 watt PSU would be better.
Thanks for saving me a post