Almost a year ago I rebuilt my pc. I went from a box that idled at 141W down to one that idled at 58W with MUCH better performance. There were two keys to the build, one was a power efficient CPU, the other was using nVidia's hybrid SLI technology which basically allows me to power down the video card when it is not needed.
I have a thread about it here:
Building an efficient computer - EcoRenovator