I am considering the 17" iMac, which is basically a laptop on a stand. SilentPCReview did a review of the older Core Duo (not Core 2 Duo) model and the most wattage they could get it to pull was 73 watts, and that was running Windows with 2 x CPUBurn and a ATI Tool stressing the video card.

http://www.silentpcreview.com/article594-page1.htm l

Another option might be to build a PC with AMD's new 65nm Athlon X2 CPUs, a motherboard with a built in GPU, a 7200rpm laptop drive, and LCD screen. You'd think that would be fairly miserly.