We have 5 Content DBs for a Web Application which has 50+ Site Collections. I want to move an existing Site Collection from one content DB to another.
Is that possible, if so how ?
You can use STSADM -o mergecontentdbs for this. You'll need to use the option where you specify a list of site collections to move (i.e. just one in your case) in a file passed to the command.
The TechNet documentation has the details: http://technet.microsoft.com/en-us/library/cc288557.aspx