I'm probably the wrong person to ask -- since, on last count, there are 7 computers in the house.
I don't know how green it is, but the box running my X10 software and the weather station is a XX year old PIII laptop at 450MHz. I guess I could easily run on a low wattage box. I don't know the specs, but I'll bet that laptop is somewhere in the 50-120 watt range.
But - since I paid $20, it would probably be a long time before I had a decent ROI, even with a low wattage machine.