As you may know MS doesn't recommend to have a content database more than 100 Gb. Keep in mind that you can use different content database among several site collection, but NEVER inside the one.
Here is easy to read article about this topic
And the hit of all season - sharepoint magazine article about sharepoint farm configuring and planing
Please refer to the post Architecture: different site collections or farms for departments? to understand when to create a site or a separate site collection
The article on MSDN Performance and capacity test results and recommendations (SharePoint Server 2010) ("Limit content database size to improve manageability" section) states:
To help ensure system performance, we strongly recommended that you limit the size of content databases to 200 GB.
A site collection should not exceed 100 GB unless it is the only site collection in the database. This limit exists so that you can use the SharePoint Server 2010 granular backup tools to move a site collection to another database if you need to.