I was curious how much power my 1.8 GHz G5 consumed when it was running and when it was asleep, so I measured it last night.
Awake, no applications running, using an Apple 17" LCD and Apple keyboard and mouse (supplied with the G5). Total power = approx. 170 watts.
Asleep, using the button on the LCD screen = 10 watts.
Total power off, still plugged into the outlet = 2 watts.
Interesting huh? Seems like putting it to sleep is just about as good as turning it off.