I have one of these, and couldn't be happier:
http://www.lannerinc.com/x86_Network_Appliances/x86_Desktop_Appliances/FW-7535
I got this sub-model: FW-7535F: Pineview D-510 CPU 1.6GHz; w/ 1 x SO-DIMM slot, 1 pair bypass, fanless
Saturating a 20Mbit/s link doing 256bit AES IPSec barely registers on the CPU scale…
I put 4GB RAM in the box, and recently replaced the CF card with a 60GB SSD from intel, which I got for $60 at NewEgg.
The thing is now fast, and doesn't have any moving parts, since it's passively cooled, too.