Copying one SharePoint 2007 web application with all site collections to another farm.
Here are the steps described under “Migrating using SQL Server Tools”
to move databases across farms.

Steps to move Site Collection from one farm to another farm:

• Create a SQL backup of the content database of the web application you want to copy/migrate
• On the destination server, restore the backup in a new database.
• Create a new web application on the destination server (use the
  default database name proposed by SharePoint)
• Detach the database
  • Go to Application Management – Content databases
  • Select the correct web application
  • Set the database status to offline
  • Backuprestore1
  • Click Add a content database
  • Enter the correct details about your restored database and click OK to finish

stsadm -o addcontentdb -url [http://Youroldserver] -databasename
[WSS_Content] -databaseserver [SQL Server Instance]

You will get   “Operation completed successfully” message when migration is done