All the documentation regarding installation of SBS 2008 is available on Technet at Windows Small Business Server 2008 .
The problem you are having is that it's pretty clear in the documentation that the SBS NIC must in direct communication with the Hardware that is providing interenet access, such as firewall/router. You can have this router/firewall and the SBS nic connected connected to a network switch. And SBS does want to be connected to the network during the install.
I would strongly suggest using the microsoft public newsgroups for help with SBS 2008. Using any newsreader, you can configure for msnews.microsoft.com and then subscribe to microsoft.public.windows.server.sbs
Hope this helps
Cris Hanna
Microsoft Small Business Server MVP
Owner, CPU Services Bellville, IL
CPU Services Home