Depends what other parts you are going to use too. But I'm think you should get something in the 550+ watts to 750 watts range.
Also, when getting a psu, you need to care more about the +12v amp numbers, this is the one that really matters.
Here are some good brands to look at, but I think most will suggest the Corsair one.
CORSAIR
PC Power & Cooling
Antec