If you create a new content db, you can assign it to new site collections that will add content to the new db. Old site collections will continue to use the old content db.
If you have a db that is too large (say 150 GB) but with enough space in it, you can trasnfer the site collection in a new content db that is smaller. In your case I believe your option is to migrate smaller pieces of content to separate site collections.
I myself like to use 3rd party migration tools to do that (such as DocAve or Quest).
Kind regards,
Margriet Bruggeman
Lois & Clark IT Services
web site: http://www.loisandclark.eu
blog: http://www.sharepointdragons.com
But how can I migrate smaller pieces of content to separate site collections because this content DB only have one site collection with many document libraries. If I move document libraries to different site collections then user won't be able to access
it easily like they do now.
By the way is there a way to check exactly which site or document library is taking the largest space in content DB?