I was thinking about doing something similar (Atom ITX with 2 nics between my DSL modem and Wireless Router)... but I was going to use Untangle.
I'll have to check out PFSense since it also has modules like Untangle.
After looking at PFSense's available packages... I think Untangle is closer to what I am trying to accomplish.
Here's a list of the Free Apps you can get with Untangle: