If you add a new content  database, and remove the old one, well you'll loose all
your data that way.
Also, if it's the Default content database  for the portal, Portal prevents you
from removing/adding that database from the Manage Databases page in WSS Central
Admin.

If you restore from a known good backup, you'll still need to detach/reattach
that database in WSS Central Admin, so that the "Site discovery" detection will
run and update the Configuration Database appropriately (in case new sites were
added since that backup, those would still be listed in the Configuration
Database)

What I would suggest, is you restore the known good backup, then run the orphan
detection operation of STSADM, but you'll need the hotfix as described here:
blogs.msdn.com/.../652453.aspx