I have seen this question pop-up in the forums and I think I also got an email from one of my blog readers asking how they can move a site collection from one database to another database.
This can be done via SharePoint 2010 Management Shell [aka PowerShell] and this is what you need to do.
You must have the content database in place [created before you do this operation] for a particular web application and then use the following
Move-SPSite “http://justsharepoint.com/sites/team” –DestinationDatabase “ContentDatabase02” -confirm:$false
Done. Your site collection is now in ContentDatabase02