850W is plenty, you really only need 550, but 650 would probably be best. However, Rosewill xtreme series is not a very high quality power supply. If you haven't bought it yet, I would suggest a better one like this:
http://www.amazon.com/SeaSonic-650-Watt-CrossFire-Certified-SSR-650RM/dp/B00918N4A0/ref=sr_1_4?s=electronics&ie=UTF8&qid=1395583887&sr=1-4&keywords=seasonic+atx