Some thoughts:
Check the following article on Support for Microsoft Dynamics CRM 4.0 on Windows Server 2008-based computers:
http://support.microsoft.com/kb/950100
If IIS port 80 is already used, install Microsoft CRM on another port (5555)
Since you are running SBS 2008, go with the Premium edition - it allows for SQL Server 2008 for running on a second server. Since CRM 4.0 supports both SQL Server 2005 and 2008, go with 2008.
I would setup a baseline SBS 2008 Premium and install the CRM on the same server as the SQL Server 2008.
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