That's a good question. I suspect its going to end up being a lot more complex than that, since multihomed DCs, Exchange Servers, etc are classically problematic and the SBS wizards do magic to remove the problems.
Hopefully someone will come in here who's jiggy on the SBS weirdardry and come up with a solution.
From: Kernersville, NC USA
Here is what you need to do. Install the second nic to use for the internet connection. Configure it to talk to your router/firewall. run the ceicw from the sbs console. Now assuming you have the other sbs sp1 patches already installed, go ahead and run the ISA setup from the sbs sp1 disk 3 autorun screen.
The installer will rerun the ceicw for you at the end of the setup, make sure to let it recreate the certificate, it will not be able to use the old one.