400 is enough to run all the latest hardware, and a few (3-4) fans comfortably. 450 would be perfect. 500 would be slightly excessive. Anything more really isn't needed, just like they say you don't NEED any more than 256MB of RAM, but with computers, more is ALWAYS better (with the exception of heat).
However, beware of cheap PSU's. If you get a 500W PSU for $40, but it's 5v average is 4.90v, then it's not worth it. Stick with the name brands, they're tried and true. Enermax, Antec, Enlight, and IWin come to mind. Stay away from bargain PSU's, they're probably more trouble than they're worth. Enermax is your best bet, if you have the money. They make solid, well made, high output, stable PSU's, and aren't terribly expensive like a redundant server-grade PSU.