I've been using something called a Kill-A-Watt (
review froogle) to see how much power my PC is actually consuming.
Interesting stuff.
If nothing else, it gives me a notion of the max heat my system might be shedding, which I can narrow down by starting the system with power not connected to the motherboard (jumper) and using that as a "base" - and fudging things a bit based on my guestimate that my PS is somewhere between 68 and 78% efficient (
PS review).
Just mentioning it here as someone else might find it interesting...