As others note, the chipset for Atom is a hog.
Just built a new home server with the idea of maybe doing mythtv too, ATI 790GX chip set with integrated HD3300 graphics, an Athlon 64x2 5050e (2.6GHz), 4GB of RAM and a small fast SSD drive for the O/S (so it only spins up the hard drives for data files) - total power draw is 45W at idle, peaks at around 80W so far, and the performance slaughters the Atom.
At this sort of power consumption the Atom doesn't really make much sense until they get the chipset sorted.
Oh, and that's witha 'normal' power supply, which is probably not too efficient at such a low power draw, so I'll swap it to a PicoPSU or similar ASAP and probably drop another 5 or 10 watts as well as moving more heat sources from the machine (but it all runs very cool on a single 120mm case fan anyway).
The surprising thing is that each (DDR2-800) DIMM seems to add about 5W !