For a 5850 you would want a quality 650w or higher with 26amps on the 12v rail and an 80+ certification , this would be ideal
http://www.newegg.com/Product/Produc...=corsair%20650
As for the 300 series not much is known , except that it most likely will be more expensive then the 5000 series. Also a good chance nvidia will just rebadge alot of their current cards as well.