Yes, nvidea reccomends 400W for the 550 and some other reviews show your total poweruse is around 300W with it. AMD reccomends 450W but i'm pretty sure thats overstated as it uses less than the 550.
Btw, if you decide to go for AMD a 6770 is a rebranded 5770 so maybe cheaper and the same performance.
Btw maybe think about a gtx460 if you're a gamer, altough the power might really become a problem. It's not a 80+ certified psu i think, so that will be to close for comfort maybe.
Nvidea reccomends 450W. On the other hand according to this it is possible ; http://www.guru3d.com/article/geforce-gtx-460-review/13
Also you have to use a molex adapter because your psu just has one pcie 6 pin and you don't have to do that with the other 2.
Still, if you're a gamer and maybe in the future wanne play at higher resolution, a better psu and gpu are the way to go imo.