A quality 550 watt unit should handle that system fine. While you may be able to get by with less, I like to recommend a bit of headroom.
The 7950 has similar gaming power use to the old 5870(impressive for the extra performance). I have had overclocked systems with that video card and saw full loads under 400 watts(at the wall).
Please ensure you are getting a quality power supply with a large portion of its overall power on the 12 volt rails because cheap ones or old designed put more of the power on the less used lower rails(3.3/5)