Just be sure to check reviews before you actually buy a PSU. Often companies will over-rate their PSU's and they can't actually handle all they say they can. However, you should be OK with a 400 watt as GVBN suggested, even if the PSU isn't actually 400 watts.
Well it all depends on the brand. For example, if it is a 350W Fortron PSU, it will do the job fine But if it's a no-name brand, then I would consider finding another. Don't go crazy though, it's not like you're actually trying to get 400w from it.