There are several articles out there that tell you how to change the SQL Server for SPS2003 and WSS 2.0. When I ran into this on a customer site, I couldn't find an article that said how to do it for MOSS 2007 and WSS 3.0. I don't have the time at the moment to write up a nice guide, so I'll just write the quick and dirty steps now.
1. Backup the entire farm using the GUI based tool on the Operations tab in Central Administration. (Make sure it completes successfully)
2. Remove all the MOSS machines from the farm
a. Remove the Incoming and Outgoing email settings
b. Using the SharePoint Product Technology Configuration Wizard, remove each machine from the farm.
3. Create a new configuration database
a. Use the SharePoint Product Technology Configuration Wizard to create a new farm, creating a new Configuration Database,
ON THE NEW DATABASE SERVER
4. Attach the rest of the servers to the new farm usng the SharePoint Product Technology Configuration Wizard
5. Restore the backup taken in step 1, using the Restore feature on the Operations tab in Central Administration.
a. Ensure the MOSS Search Service is started on the farm
b. Ensure the Windows SharePoint Services Administrative Service (Windows Services) is started on the farm
You can also do this with STSADM, but again, that might come in the form of an update to this post. Here are a couple articles that have some pointers if you're having issues doing this.