I've seen a few to date, but this one is simply impressive in the breadth and level of details covered. Besides overclocking, you can factor in Peak Utilization, Capacitor Aging (Explained at the end of the page), and Surge Compensation which deals with the power surge required when you first power on the system from a cold start. From my experience trying to calculate what my main rig draws, the number given was very accurate.

http://extreme.outervision.com/psucalculator.jsp