I regularly build office machines for my wife's company. I try to reuse existing equipment whenever possible (mostly floppies, Hard disks, and cases). When reusing a case, I have to guess if the power supply is strong enough to handle the new motherboard/processor I'm going to stuff in the case.
Can anyone suggest a way to calculate the minimum power supply I need for a given processor/MB/video?
Keep in mind when you re-use a case, that the PSU with it may not be capable of putting out any more than 50% of its label claim without choking, smoking, and croaking. Fortunately, the power requirements of most business machines are very low, so a "500W" PSU that is really only capable of 250W may work fairly well.