IIRC, Pfsense doesn't require a whole lot of hardware.
I'd probably just do it on the cheap, and pull some Intel PCI network cards off the bargain pile. They should be supported out-of-box.
I'd go as low-voltage as possible to keep the heat down for the rest of the box. I'd probably go with a Wolfdale Pentium dual-core, probably the cheapest one, and an inexpensive micro-ATX motherboard to support it. Even if the board has an onboard network port, it is probably Realtek, so I'd probably still go with two Intel PCI network cards for best driver support and performance.
This Biostar mainboard looks like the perfect candidate.http://www.newegg.com/Product/Product.a ... 6813138314
Intel's Celeron E3400 is the second-cheapest Socket 775 processor at NewEgg (the cheapest is single-core, 65nm rather than 45nm, and is only six dollard cheaper, so I'd skip that).http://www.newegg.com/Product/Product.a ... 6819116348