The Micosoft article, "How to Promote a Management Server to a Root Management Server Role in Operations Manager 2007" does a pretty good job of explaining how to promote a SCOM 2007 management server to a root management server.
While performing a disaster recovery test today, I found that I was getting the following SQL exceptions when I ran the ManagementServerConfigTool.exe PromoteRMS command:
The type initializer for 'Microsoft.MOMv3.Setup.MOMv3ManagedCAs' threw an exception.
Turns out this is because I ran the ManagementServerConfigTool.exe PromoteRMS command directly from the SCOM SP1 Support Tools folder, which is missing some of the DLLs required to run the command.
Simply copy the files from the Support Tools folder on the SP1 CD to the local \Program Files\System Center Operations Manager 2007 folder and re-run the command.