As SG and Owen pointed out, your internal external NICs MUST be in different
subnet if your physical setup is per the diagram at:.
In general, the easiest to change is the external subnet. You can leave
your internal subnet at 192.168.1.x but you must change your external subnet
to something different (like 192.168.16.x, 192.168.2.x, 192.168.100.x,
etc.). (See the difference in the 3rd octet)? This means changing the
subnet information on the external NIC and the LAN side of the router. And
let the SBS server handle DHCP (if you don't already).
Again, follow the diagram above and use the wizards.
Merv Porter [SBS MVP]