Although each content database can contain multiple site collections, each site collection should be assigned to a separate content database for an end-to-end extranet backup and restoration solution.

To make this one to one assignment of site collection to content database in the SharePoint Central Administration site, create the content database first. On the content database creation screen, set Number of sites before a warning event is generated to 0 and Maximum number of sites that can be created in this database to 1. Then, create the site collection so it occupies the only available spot in the content database.

Central Administration doesn't currently allow you to specify a content database on the site collection creation page. However, you can use the Stsadm console utility to create a site collection and a new database simultaneously by using the stsadm -o createsiteinnewdb command. If you choose to use this command, you'll still have to go back and edit the content database and set maximums. You'll most likely be on that screen anyway since this script doesn't have an option for selecting a search indexing server for the content database. Considering you want to be able to search documents by default, the only way to automate the entire process outside of the API is to use the following series of stsadm commands. 

• “stsadm -o addcontentdb …” creates the content database with the maximums
• "stsadm -o createsite ..." creates the site collection assigned to the content database
• "stsadm -o spsearch ..." assigns the content database to an indexing server for the WSS search service (not the same command as MOSS search)

Database Management
Based on guidance from Microsoft and the recently released scalability and performance case study, you should try to keep the number of content databases under 100 per web application to maintain a well-performing environment. This means if you have more than 100 active site collections (extranets), you'll need to start sharing content databases between site collections.

Anything but the most basic of extranets should be using a dedicated SQL Server. From a SQL Server perspective, you should remember to presize your databases and choose a more appropriate autogrow interval then the default setting. The sizing defaults for SQL databases actually have a negative impact on performance in SharePoint.