I recently swapped out my router (a PIII) running Smoothwall with pfSense. I'll say that I'm glad I switched. Anyway, what I found is that Smoothwall wouldn't let me add another LAN interface, making it a real router.
So, once you get pfSense installed, go to Interfaces->(assign)
Assuming you already configured both your LAN and WAN interfaces, it should be clear which interface to choose for the extra port.
Then go Interfaces-><interface name> in my case, OPT1. Enable the interface, change the Type to Static (static worked for me), change Bridge with to LAN and Gateway to your gateway address (ie, 192.168.0.1)
Apparently, I can't read and didn't read the small print at the bottom. I forgot to add the proper firewall rules to make it work. Go to Firewall->Rules, click on the new interface tab and configure appropriately. To make it exactly the same as your LAN, change the source to <interface name> Subnet.
This is a somewhat short tutorial. If you're using pfSense, you're probably already know what you're doing. Now I can eliminate the switch I was using and go directly to the router.