I have an Atom in the form of a D945GSEJT with an Intel SSD to make it all fanless & perhaps less likely to suffer from drive failure. It works fine with WHS, aside from the initial install being painfully slow. I used it for a month without any problems, until I tried to do other stuff with it, at which point the HS aspect broke and it just became a W. WHS is very delicate - once it's set up, don't touch it. Note that WHS is normally only supplied by OEMs as part of an appliance, but you can get it online on its own for about £80. A ~£213 TechNet subscription also includes it, for "testing."
It now runs Server 2008 - note that it can't run R2 as the CPU doesn't have 64bit extensions. It can stream 1080p fine - 1080p doesn't really demand much in terms of network speed.
For a full & incremental backup system, in place of WHS, I intend to use
ShadowProtect when I eventually get around to it.
If you go for a NAS, remember: RAID is not by itself a backup system.