1. Make sure the existing MOSS 2007 farm is upgraded to the latest service packs and all system issues are resolved. Doing a migration to SharePoint 2010 won't make any issue magically go away. In fact it could just complicate matters.
2. Make the content databases in MOSS 2007 read-only. To set a database to read-only in SQL Server 2008 you open SQL Server Management Studio and use Object Explorer to connect to an instance of the Database Engine, expand the server, and then expand Databases. Select the database that you want to configure to be read-only, right-click the database, and then click Properties. In the Database Properties dialog box, in the Select a page section, click Options. In the right pane, under Other options, in the State section, next to Database Read-Only, click the arrow, and then select True.
3. Copy the .MDF and .LDF files for this database over to the new SQL cluster for the SharePoint 2010 farm in order to implement the content database for SharePoint 2010. In Windows Explorer, browse to the location of the .mdf and .ldf files for the content databases. Select the .mdf and .ldf files for the database you want to move and either copy or move them to the destination directory. In SQL Server Management Studio, open the source instance of SQL Server. Right-click the Databases node, point to Tasks, and then click Attach. In the Attach Database dialog box, browse to the location to which you transferred the .mdf and .ldf files, select the .mdf file for the database you want to attach, and then click OK.
4. Once the database has been restored then run the following command to configure the SharePoint farm.
stsadm -o addcontentdb -url
5. Be sure to do a IISRESET because otherwise it'll look like garbage until the web server resets.