Rosewill power supplies used to be junk but they have really turned it around in the last few years. They are still not a brand I recommend across the board yet as they still have a few less than stellar models but the Hive is a good one.
You can count on Corsair, Seasonic, PC Power and Cooling, XFX, Silverstone, Enermax, OCZ and Antec pretty much across the board for quality models.
A 500w power supply is plenty for a single 6870. If you plan on adding a second card then go for 650 watts.