Hello,
It is sufficient to restore the portal in case of a disaster recovery. I would recommend you to also do backups with stsadm for your sites, doing it that way will allow you to restore single SharePoint sites/site collections.
You can easily create a script that runs the backup commands and you just make it a scheduled task on your Windows Server.
Download the Administrator guides for SharePoint from Microsoft, there you will have information on how the stsadm tool works, and what options you have.
If you only do an SQL backup, I do not think you will be able to restore individual sites if for example someone deletes a document library.
We are using 3rd party backup software to do real itemlevel backups of everything, so I can restore a single item in matter of minutes.
Regards,
Thomas