http://www.hexus.net/content/item.php?item=8188Since 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 about 100W itself, it's obvious that you're quickly going to run out of power if all you have available is a 500W PSU.