Which model, specifically of PSU do you have. I was just at the AOpen site, and there are a couple of 500W PSUs that have split rails and don't even get 20 Watts on either 12V rail. This would make it unsuitable for use with the 6850. So, specifics are important here.
Going by Neweggs PSU calculator, you need 447 W, assuming 1 optical drive and 1 hard drive. I would round up and buy a minimum of a 500W PSU. Here's a couple of recommendations for a good, but budget PSU: