AD users/groups backup

If you are planning to run the two domains in parallel, try the Active Directory Migration tool. Fomr what I remember of using it in the past, you'll need to set up a trust relationship between the domains and create certain user accounts to facilitate the migration.

You can get it from here:

http://www.microsoft.com/downloads/details.aspx?FamilyID=788975b1-5849-4707-9817-8c9773c25c6c&DisplayLang=en

Or if the two domain scenario is not an option, you can use LDIF.

http://www.microsoft.com/technet/prodtechnol/windows2000serv/technologies/activedirectory/howto/bulkstep.mspx

You can also run the windows backup tool. Quite fast and easy.