650w power supply is good enough for any computer without sli videocards honestly. i'm using a 500w power supply with a gtx 460 overclocked (my own settings) a phenom II 965 overclocked at 4.07ghz 4gb of ram a sony dvd drive and two hdd's

i also have 3 case fans and two case light strips

even if you're going to run crossfire with the ati 6850 i would suggest a 650w power supply