OC Graphics Card

I'm planning on getting a new graphics card with my current build. The graphics card I would like to get is the Asus gtx 550 ti, but the only one they have at Fry's is the OC version of it. My current PSU is 400w and I was wondering if the OC verson of the gtx 550 ti would be able to run with the support of my current PSU. I know for a fact that the non OC'd version of the gtx 550 ti can be supported by a 400 w PSU. How can you tell how many watts supports your Graphics card, because some sites do not give out the complete specifications and requirements? I really need some feedback or help. Thanks for your time and consideration.