There seems to be a lot of power paranoid people around now, i'ld be willing to bet a good 250 watt psu could power his system (unless he overclocks). A 400 watt would be more than fine as long as its any quality at all.
I do agree with you. Just like anything, it is better to have an overhead of power as it helps maintain stability and consitency of power. If going cheap, getting a 600W PSU for a 250W system is a good idea since cheap PSU's have cheaper regulation, caps, etc. If going for a better PSU, a 400-500W PSU is still a good idea. Thinking of the future is also a good thing also, if you get a PSU with more overhead, you won't have to get another PSU if you want that video card or quad core CPU, or extra HDD, you spent the little extra now to not worry about it later.
I also believe that having at least a 100W overhead over system specs wattage ratings is a good rule of thumb, if OC-ing make sure you have 100W over the ratings of your OC'd system. There are many calculators out there. My Corsair for instance is a better PSU and should last me a long time, even though only 520W, I will never run SLI/CF because I'd rather have one more powerful card. But everyone has different rules of thumbs, I've tried quite a few different PSU's and I've found cheap ones need replaced within 12 months, mid-priced are a crap-shoot, some are great, some are just overpriced cheap ones. But going with a good brand one for ~$100 is the way to go in my mind. It all depends on the needs, how stable the person needs voltages, if planning on OC-ing or filling the tower with HDD's, add-on cards, etc...