1. Read this article, and then consider whether you want to even bother with an atom or Clarkdale. From an energy efficiency perspective a low end Sandy Bridge can't be beat.... even when compared to a dual core clarkdale like here:http://www.xbitlabs.com/articles/cpu/di ... html#sect0
2. Wait two months and there will be dual core plus dual virtual core Sandy Bridge CPUs that will be even more impressive energy wise... for even less money. And there is a real low end one with two cores and no virtual cores... the 2.2 Ghz G620T.
The idle on the 4 core i5-2400 is only 16 watts compared to 28 watts for the dual core i5-680 Clarkdale. The the forthcoming dual core plus dual virtual core i3-2100 should be even better. But the dual core 2.2 Ghz G620T should really take the prize for draw at low idle.... while still delivering respectable performance under windows.
Get the G620T. Pick out a low energy draw SSD; a low energy draw fan (scythe gentle typhoons are exceptionally energy efficient fans) and a low energy draw motherboard (Intel motherboards are often good in this respect).... and you should get down about as low as anyone.
3. I have never gotten hibernation or sleep to work properly on a desktop computer. It's just too fussy. On a laptop (or an Apple) they are able to fine tune the hardware to the software... and it always seems to work there. Maybe you will have better luck than me, but I have given up.
4. My recollection is that electricity cost is approximately about a dollar per year per each 24/7 watt. See:viewtopic.php?f=16&t=61255&p=531777
5. When you start getting down to low watt CPUs the motherboard starts become as important or even more important than the CPU. Intel has a reputation for highly efficient motherboards.