As of yesterday, I have installed pfsense on my routing-pc as well.
It's running on:
ASRock G31MG-S Socket 775 mATX (GBit-lan onboard)
1.8 GHz Intel Celeron D 430 (passively cooled with a Sharkoon cooler, don't remember the name, though)
2gb DDR2 Corsair Dominator (they were unused otherwise)
LevelOne GNC-0105T Gigabit PCI Ethernet Card
EW-7128G Card (54) WLAN PCI Card
all in a asus vento s6 ÂµATX-case that came with a (suprisingly silent) 250W-PSU. So currently, the only fan is the 80mm PSU-fan.
It's used mainly for gaming, but also for remote-controlling work-pcs.
The reason I got it was partly, because I had some trouble with my USRobotics router, but also because i just liked the idea of trying it out.
There is one gaming PC (running boinc) connected via LAN, and (mainly in the evenings) two additional laptops and the occasional smartphone connecting using WLAN.