Actually you can. PC power supplies use switching regulators. Switching regulators are much more efficient than linear regulators, but they don’t regulate well unless they have a certain minimum load, which is typically about ten to twenty percent of their rated maximum. Some power supplies apparently have an internal load that they will switch on if you don’t draw the minimum required current, but some will either shut down or just won’t run properly.
For PC supplies though, you aren’t going to find one commercially available that’s big enough to cause you problems. Maybe if you get one of the largest power supplies available, and then got one of those really low power server motherboards. Other than that, I don’t think it’s possible to screw it up.
The power rating is the maximum. You can think of a switching regulator as something that turns the power on and off really fast, so that the average is just what the computer needs. The less power the computer needs, the more the regulator keeps the power “off” and therefore uses less overall power. In other words, if your comuter only draws 200 watts, the power supply only produces 200 watts, even if it is rated for 350 or 400. (actually if you want to nit-pick, it would probably produce about 210 watts, since it’s not 100 percent efficient)