Since the release of NVIDIA's top-of-the-range G80 graphics processor, we've had graphics cards that can drain 200W from computer's PSU - and demand 400W if used in pairs.

Since the kind of CPU you'd use on a PC with a pair of G80s will consume ab out 100W itself, it's obvious that you're quickly going to run out of power if all you have available is a 500W PSU.