NT 4.0 to SBS 2003 migration

Hello

I am currently trying to migrate NT4.0 to SBS2003. I am confused on one issue, When I install SBS 2003 on the new server do I assign the same domain as the NT4 box? I was under the impression that SBS can be the only DC on the network in a single domain. I want to be able to use the same user/computer account so I can keep the users desktops. I read Microsofts migration plan but it is vague on the domain name for the new box. Also if anyone has run into some problems that might be of help to me I would appreciate it

Thanks

Dale