Not so long ago, a 500W power supply unit was enough to run all but the most exotic of home PC systems. But, as Bob Dylan once sang, times they are a changing.
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 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.
If you're not interested in playing safe you could buy a second PSU to power your drives and one of the graphics cards.
However if you do overload one of the supplies and, in its death throes, it creates a power spike, you could end up seriously out of pocket.
What you really need for a thoroughly-modern, high-end PC with twin graphics cards is a PSU able to put out 750W or more.
But if you've already looked at such boxes, you'll know that they don't come cheap. So, the last thing you want to do is buy an expensive PSU and find that it doesn't deliver the goods.
We've check the performance of five of these wallet-sapping beauties on a professional PSU test system. If you want to know which PSUs live up to their ratings and which fall short, read on...