The Official SBS Blog : Requirements for Installing SQL 2008 Standard Edition for Small Business:
To follow up on this post, some other issues you need to be aware of if you attempt to install SQL 2008 ON the same server as the SBS 2008. (Keeping in mind it would be best if you put the SQL on a second server or a virtualized server rather than on the SBS itself)
One thing you will have to do is remove the SQL 2005 management tools from the server and then install the SQL 2008 management express tools. You can’t load the 2k8 Full server tools on the 2k8 box.
Cannot install SQL Server Management tool on Windows XP or Windows Vista platform using the SQL 2008 DVD included in SBS 2008 premium edition:
Microsoft SQL Server 2008 – Installation Made Easy
Windows Small Business Server 2008 Technical FAQ
“Can I install SQL Server 2008 from Windows Small Business Server 2008
Premium Edition on the first server?
Yes, this is a supported scenario. However the SQL Server 2008 management
tools will not install on the same server; you must install them on another
If you haven’t yet installed .NET 3.5 sp1 it will install that first.
Then it will install the 4.5 installer
Two warnings you will get as well
As I said, consider that you are putting it on a DC.
Then only pick those specific pieces you need. Check with the vendor to see exactly what parts you need and install only the minimum.
And don’t forget you’ll need to remove the SQL 2005 express tools before you can install it “ON” the server.