My rig pulls 283 watts at boot with a 3.4GHz quad and a GTX (now a GT havnt tested that yet) That includes 4 hard drives and a crap load of fans. load hits high 300's but a 550W is still capable of powering it. grab a kill a watt meter there like $20 dollars I think home depot carries them
Oh and those power draws were measured at the wall so factor in 80% and your only using 260 at boot and maybe 350-375 load