load balance ordinary traffic

[Justin C. Sherrill]

I have a FreeBSD 4.8-stable machine with multiple network interfaces.  One
goes to a local network (192.168.*) and the other goes to the Internet via
cable modem.  This machine does NAT for the 192.168.* network.

I'd like to add more Ethernet cards to this computer and connect them to
other cable modems, thereby providing more bandwidth to the 192.168.*
network.  Where can I look for more information on how to do this? 
Specifically, the software changes (with ipfw?) to "concatenate" the
connection.