Have you considered/compared the new MacMini?
Uses about half the power on idle (<10watts)
Has Core2Duo processor (so better performance than Atom processors).
Hard drive rather than SSD.
May cost a bit more.
(I have no experience with either one - but have been shopping/looking at specs.)
If an atom system looks like it would supply your needs for the next 5+ years, then might be reasonable.
As noted in another thread http://www.silentpcreview.com/forums/vi ... hp?t=59331
unless really tight on space or maximum power, atom systems
don't seem to make much sense at this point.
You can get not too different idle power (especially if you undervolt) with better performance for less $ from Core2Duo or i3 system, admittedly with a fan. Since much of energy use is in making the computer, I lean towards something that likely to meet needs for longer, be repairable/upgradable/etc.
edit: And of course getting a second hand Mac Mini, or other system would be even greener. (Older Mac Mini's used more power than the new ones, but still less than the H1, if memory serves.)
But a lot depends on your needs.