The issue with those recommendations is that even the least conservative ones are still pretty conservative about power. 400 is not ideal but I doubt his system will be pulling more then 300 watts under load, probably closer to 200.
If your system has a cheap no name Chinese crap PSU then you may want to consider picking up a Corsair CX or CXM 450, they are on sale for usually around 30 USD. (make sure its the model with white and black lettering, not the green, the green lettering ones are junk)