Dell has come out with several new green desktops and servers. The Optiflex 740 and 320 are supposedly more energy efficient than most, though I can't find the documentation to support it. Here's their site, you give it a whirl.
Dell has also come out with a few server lines, the 1950 and 2950 lines, that have similiar claims. Clearly, the processor uses less energy, but the machine is also less functional. And it costs more.
C'mon, this is a three cylinder offering, as good or as bad as any other product. As that biting realm of sarcasm the Register has observed, "All of the hardware makers look to capitalize on rising energy costs by pitching themselves as environmentally sound sellers of plastic, metal and chemically-enhanced computing innards." How about putting Linux on it and extending the warranty to five years? Try harder Dell.