If this were known, the best option would have been to purchase Windows SBS 2008 PREMIUM rather than standard. This would have allowed you to use SBS 2008 as the domain controller and run Exchange Server 2007 and then implement the free Windows Server 2008 Standard as your add on server on which you could run SQL Server 2012 Standard or Express.
This is the method which Microsoft recommends. It is clearly recommended not to be on a domain server and, therefore, not on SBS 2008.
Having said that if your domain is in the neighborhood of five to twenty clients, you would likely have enough server to run both Exchange and SQL.
