The beauty of pfSense is it's flexibility. You can maintain your existing networks and then have pfSense route traffic between them. You could build two separate VLANS and have pfSense route between them. You could build one big network and have pfSense manage a primary Internet connection and then automatically fallover to the secondary connection. You can have pfSense send certain traffic over one Internet connection and other traffic over the second connection.
If you can imagine it, pfSense can probably do it. It's an awesome tool.
A broad overview, w/ horrible sound for first three minutes: