Just my 2 ct
Originally Posted by stevea
I dodn't have experience with headless systems yet, since I work (yet) only on x86/amd64 actively.
I don't know it arm and the others have CPUs capable of doing the jobs you want.
What I know is that the 4850e/5050e from AMD is a fine choice, it's cheap, you have a lot of Mainboards to choose from (ASUS M2A-VM ist quite nice and cheap, too) it's quite low power, passively coolable (I did exactly that), and still provides CPU power when you need it.
The other possibility I see is the mentioned VIA with padlock. The padlock part is nice the rest... huh. Well. I don't have a high opionion of it. Okay, I must admit power consumption with the right VIA ones can be really low and of course also passively coolable, but they all lack raw computing power (besides the padlock) and the drivers are often an issue. But then you didn't say you wanted graphics but since this is the main problem with drivers it might still work for you. Oh and sometimes something is broken on these mini ITX boards. In my case the HPET was in an unsuable state.
I don't know about intel but these atoms aren't that great as far as I heard.
If you go for a non x86-arch you would have to find a (meta)distribution that works on the specific arch and you won't have such a lot of packages to choose from. Gentoo e.g. has a package list comparison chart and you clearly see that most packages are x86 > amd64 > ppc and then all the others and embedded ones. On the other hand systems with power uptake less than 5W are possible.
A generally good idea would be either laptop HDD or flash memory built in. A single block of RAM (but large enough to avoid swapping or such things) and on x86 you sometimes have an undevolting option in the BIOS which can save you further watts - but check for stability.
Stop TCPA, stupid software patents and corrupt politicians!