You can use standard SharePoint OOTB utility "STSADM". This can be found in 12 folder (C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\BIN for standard installation path).
Use restore/backup command to backup and restore entire site collections. This will do full restore of the site collection. Do not forget to change site colletion administrators when restore is done.
Or you can use export/import command useful more for content migration without affecting user permissions and settings of the site collection.
These links can be useful:
Hope it helps,