Heya,
Just as a side note, when building, you can also undervolt and underclock the CPU to drop some juice from it. With CoolNQuiet also throttling back on top of that, you can keep it low even when it picks up a tad during various tasks. This way you can get a relatively lowered dualcore that is more than capable of doing the server's work, but without basically being a mustang engine in a parked car. I took a Regor 2.9Ghz for example (65w) and undervolted it a bit and underclocked it down to 1.6Ghz. Didn't change performance at all on the server. But it dropped the power usage and lets me go fanless on the CPU (large heat sink instead for passive cooling), no fan on the CPU = less power usage as well. Some of these fans can eat up 5~7 watts on their own all the time. At this point, you'll find that you can't get things to drop lower than a certain value regardless of what you do, and that's where you find the power usage of the actual motherboard's chipset which you won't be able to do much to lower, if any. And that's where a lot of the Atom based boards are doing wrong--the Atom is low power, for sure, but the chipsets and the boards they're put out on are not, and that's why you see these weird 40+ watt draws from an Atom board when the CPU may draw like 7~10watts. That motherboard chipset does three times the CPU's consumption. So what I'm getting at, is you might as well get a good CPU that can do real work (single core sempron outperforms the new Atom dualcore, weird). And tweak the board/cpu to be low power.
Mine (the Regor) has webcam surveillance software running at all times with a camera powered through its USB port (2.5watts if I remember correctly) and 5 HDD's (all WD greens except for the OS drive which is a WD Blue), FTP, uTorrent, and is on a UPS. It draws 55watts at idle, but even when I'm copying data or actively seeding, it doesn't really budge from that point.
Very best, 🙂