Get ID for a SharePoint 2007 Database

One easy way of figuring out the ID of your SharePoint 2007 db is to mouse over the content database in central Admin/ Application Management
Copy the shortcut for the link
Paste it and look at the string do not worry about the first two characters

Moving a Site to another content database in SharePoint 2007

First you have to generate xml file that contains information about sites and content dbs.

Use this command

stsadm -o enumsites -url http://websername >C:\sites.xml

to create sites.xml

Clean it up and only leave site you want to move

Rename to move.xml to only have info about the site you want to move

Now run this command

stsadm -o mergecontentdbs -url http://websername /sites/sitename -sourcedatabasename Old_Content -destinationdatabasename New_Content operation 3 -filename C:\move.xml