How things have change over the last couple of years with the computer power supply. It used to be that 500W was all you needed to power your system, but today we find 1kW and higher power supplies in many enthusiasts' systems. Of course, this is all because of the SLI and CrossFire technologies that have taken off since they first appeared. It's is possible to find many systems with two graphic cards or even three graphics cards, and sometimes the ones with only two cards each consist of two GPU cores, bringing the total to four GPU's. These high performance graphic cards draw insane amounts of power.
Yes, average users probably will find a 400~500W power supply more than adequate for their needs as today's components are far more efficient than they were before while at the same time performance is much better than it used to be. Still, as long as the industry keeps pushing for faster components, we may see the power demands for computers go even higher.