Getting an energy-efficient CPU is the first step when you´re trying to build a low-power file server, NAS or whatever you might call it. A good choice for that task is an Intel Atom. Unfortunately there is no real choice about a good mainboard for the Atom. You can either get one of those old Intel chipsets where the chipset wastes more energy than the Atom or you get one of those nifty Boards with nVidia ION platform. They´re impressive, fast and... cost a lot of money. So you better have a look at AMD.
