I agree the PSU rating is the maximum power consumption. It's even worst for the underated power supplies where-in the sticker shows 600watts but in reality has a maximum of 300watts only.
Here's something I have tested that might interest everybody: mini power meter.
This measures the actual consumption of your computer or any device connected to this in realtime.
In fact you will see a drastic increase in power consumption from idle to when you run games.
This same device when I tested it with an Intel D510, 500GB with 2GB system it only registers 35Watts excluding monitor.
While a strip down Intel Pentium 4 - 2.4Ghz with builtin videocard, 2GB, 160GB consumes as high as 150Watts.