Rod has a good link there - but I think the calculated values for overclocked CPUs are wrong. I appear to have another way of getting the wattage from on OC CPU - in the case of my overclock below - I would estimate my CPU is using 116W instead of 110W. Either way, the other drawback of this form of calculation is that you will likely NEVER reach the *maximum* figure this page suggests under real-life conditions. Combine that with the fact that most PSU specs are exagerrated to begin with. (i.e that 800W PSU can't really handle 800W). I think kd2777 had it about right - go with a brand name 500 Watt-er, and you should be fine for a typical workstation PC.