A good rule of thumb is to always have 30% "reserve" in your power supply, at least on a high end gaming rig. Here are some reasons:
1) You want it to work on the hottest day of the year, where its output could be a lot less than in ideal conditions.
2) Capacitor aging can decrease the output...
LOLright? :P in grad school I carried a Gateway FX 17" everywhere.
Although even really strong people have to watch how much they regularly load their lower backs...