The problem with the MOSS Search database is that its very tightly coupled with the index file that physically resides in the file system of the farm's Index server; I believe transactions are sync'ed down to the millisecond. So if you lose your Search database your only option (unless you have a specialized SharePoint DR tool) is to rebuild your index and start over with a new Search database, because your index file will be out of sync with the restored database and become corrupt.
The latest version of Microsoft Data Protection Manager 2007 is able to back up the search index and database, but you have to run a special script to enable that functionality. I'm not sure if tools from other vendors are able to do it, I think that several are but can't remember off the top of my head. The only way to restore your index if you're using SQL backups or SharePoint's out of the box backup/restore tools is to rebuild it from scratch.
The previous answer is spot on about managing the size of the search corpus with multiple indexes, although it does add some additional overhead and complexity to the farm. An additional index server will need to be build and it can be a challenge to effectively manage user queries so they hit the correct index and/or merge results.