I don't know whether it's the same for SBS2003, but for SBS4 (the NT4 version), you could only run one SBS server on the domain. This is because, on the NT version, it expected to be the PDC, and you couldn't demote it to a BDC or member server.
I would presume that MS have built a similar constriction into SBS2003. It might be easier to buy a normal version of Server2k3 - since it's only going to be running an intranet, have you looked at the Web Edition?
Rgds
AndyF