Energy Efficient PC's?

eebly asks: "I'm working with a team at my school to help ensure that some of our new building projects are environmentally friendly. As part of that, we want to include good, energy efficient computers. The new building will be labs, so we can't sacrifice power. We're looking for suggestions on what machines use low amounts of power, techniques to make the machines run with less, and ways to tweak the software to make things like sleep mode get used more effectively."