I setup a small system for Sab awhile ago now.
Its based on this board, a Zotac NM10-DTX WiFi.
http://www.tweaktown.com/reviews/3317/z ... index.html
It has the new pinetrail Intel Atom built onto the board, which is a dual core Hyper Threading (4 logical cores).
I have 2GB of RAM installed, 2 500GB HDD and 1 2TB drive.
The board has 6 SATA points (main reason I bought it.
So its small (DTX is kinda between ITX and ATX)( will fit any standard ATX case)
It uses very little power and the Atom CPU doesnt even need a fan.
Its Cheap, amazon currently have the board for under £100 (so about $5 or $6

)
I have it running Windows Server 2008 R2.
Its runs sabnzbd very well, unpacks and even does par2 recoveries without any fuss.
Also runs SickBeard, Dropbox and TeamSpeak all at the same time without any issues.
The CPU cores may not be the fastest, but the fact that theres 2 of them, each supporting Hyper Threading makes it a very attractive CPU
I know most people would think Im a loony for running windows server 2008 instead of linux, but I feel its easier, easier to quickly run a small tool or other apps whithout hassel and I already have a legit license for it.
Of course Linux could be run just as easily on it, although apparently the SATA Raid options are abit more limited, but I think the latest Bios update fixes that anyway.
If you or anyone else want to know more just ask.