A good quality 380-450 watt unit would be plenty and still allow for upgrades.
Here's a few options from Antec but be warned, they do not come with a power cord.
If you need the power cord, here's a couple of Corsairs
A 400 watt Corsair would work if you know that you will never upgrade. I like the 400. It is one of the best sub-500 watters around. I do not particularly like the 450VX. It's undoubtedly a nice PSU, but with only 10% more usable power (33 amps @ 12 volts instead of 30 amps), I don't think that it's a particularly good deal.