I built a box two months ago for Untangle, which is very much like pfSense.
I built it for ~$400.
Popped in an Athlon Dual Core at 3.2ghz (Athlons are plenty fast for this workload) with 4gigs ram, and two Intel NICs. I like the Intel NICs because not all other brands do checksum offload (which can help performance quite a bit), and tend to have better driver support. Untangle really likes Intel NICs.
If you have the choice, I'd recommend going with a faster clocked dual core then a lower clocked 3 or 4 core. Unless you have a huge load, then go with a high clocked quad core.
Also load it up with ram.. I'd recommend 4gigs at a min since its so cheap, and you want the system hitting the HD as little as possible. Grab a cheap HD, but a decent case. We reused an old case with no fans and its too hot in the box, so it locks up. We have a fan in there now which has resolved the issue mostly.
Hope that helps.