AMD does recommend a 500w PSU and even though what really counts is what the amps on the 12v rail is. But since you need a PSU anyway the 500w is the best way to go.
So the brands you should look for are Corsair, Antec, Seasonic, XFX, Silver Stone, and some OCZ PSU's. I will link you too some PSU's and a list of PSU's that go from the best Tier 1 to the worst Tier 5.
That way if you can't find these brands you can look on that list and know what not to buy. The 6850 is a nice card but the 6870 for a few dollars more is better. I will link you to that as well. Good luck to you.http://www.newegg.com/Product/Product.aspx?Item=N82E16817139027http://www.newegg.com/Product/Product.aspx?Item=N82E16817371035http://www.newegg.com/Product/Product.aspx?Item=N82E16817139033http://www.eggxpert.com/forums/thread/323050.aspxhttp://www.anandtech.com/bench/Product/291?vs=290