X-bit Labs investigated how much Watt computers really need, they took a look at office computers, home PCs, file servers, and three gaming configurations. You can check out the results over here.
Choosing a power supply for a specific computer configuration is a kind of an eternal problem, especially when the configuration is too advanced for the typical 300-400W PSU bundled with the system case to cope with. You can take it easy and buy a PSU with a wattage rating of some 1000W, but this is likely to be a waste of money. Unfortunately, there are often no comprehensive data on the power requirements of specific components. Graphics card and CPU makers want to be on the safe side and declare overstated specifications. Various power consumption calculators use obscure numbers. And the measurement of real-life consumption, even though mastered by most computer-related media, is often deficient..