I'm running a mini-itx board with the nvidia ion chipset and an 1.6 ghz atom cpu; the box is the size of a book and has nothing in it except the motherboard. It consumes typically 25w. I use windows 7 on an external ssd, and crunchbang on a notebook 2.5 sata drive in a drive enclosure, and tiny core from a usb stick.
I like tc because it runs in ram [ I have 4gb of ram] and the write-tos are kept to a minimum [my sticks have gone two years so far and counting]. It's pretty fast once loaded and really keeps the power consumption to a minimum [just powered off a usb stick].
I am a bit weird about energy efficiency - still lately my electric bill has been less painful reading!
I have a big box with the usual gubbins in it but it makes a sound like a hoover and generates enough heat to heat a small room. I don't use it much. The small PC is fanless and silent and that's how I like it....