Gow,
If all clients use the SW as the gateway, and the SW gateway is your DSL router... Firewall is achieved as all traffic must pass through the smoothwall.
We may be talking about different products here, I am referring to smoothwall ie
http://www.smoothwall.net. Maybe you are using a free version?
I opened a ticket with Smoothwall support about three years ago and their technician told me that the system was designed to run off one NIC. If you want the actual copy of the e-mail I can try to dig it up