There was a "Transition Pack" that allowed you to do that. It changed your licensing from SBS 2003 std, to server 2003, exchange 2003 (standalone), and allowed you to upgrade your CAL's. The cost was basically the difference between what you paid for SBS and your CALs vs the cost of buying it all separately. Where you wouldn't need Exchange 2003 I don't know that it would be cheaper. If you premium it addressed SQL as well. The transition pack is discontinued because it only converts to 2003 and not 2008, but you may find some retailers with it.
Personally I would buy a server 2008 Volume license with downgrade rights. This would allow you to install a previous version such as Server 2003 R2 std, 32 bit, but be able to use migrate the license at a later date to server 2008 at no cost.
