Some readings
> although brightness setting will have some effect
More than you'd think. We measured one of the 24" 1080p Dell LCDs we have at work the other day, and it varied from about 35 watts on the lowest brightness setting (practically unusable), to about 55 (optimum) and 80 (highest brightness).
> Plasma displays, however, can tip the scales at anything up to 500-600 watts
Only if they're really (REALLY) big. I have a 42" Panasonic, and it measures about 150 to 250 watts, depending on what it's showing (unlike an LCD a plasma will use less power to show an all black screen than it does to show an all white one).