The FIRST step before migrating is to take a complete backup of your server including the system state.

The best way to migrate is mentioned in Microsoft's KB884453, the link to which is mentioned below:

http://support.microsoft.com/kb/884453

Though this article talks about migrating a Small Business Server 2003 to an existing AD domain, however, it is the same procedure if you are migrating from a 2000 to 2003 server or from 2003 to another 2003 server.

cool stuff dude