350w or more with 20+ amps on the 12v rail by a solid manufacturer would be plenty-- unless you have a buncha hard drives and other stuff, then you might need 400+ watts.
Also, a good manufacturer's 3-400w psu will outperform a 600w!1!1 psu by a lesser brand anyday. A rule of thumb is to think quality manufacturer like seasonic, antec, fortron, tagan, with good specs-- not glowing lights, chrome and high watts.
I like to recommend the Xclio 450W and the FSP Fortron 400-- both solid, and cheap at $50.