One of the first things you are going to want to do in such an environment is clean up and "flatten out" the NT domains you have before moving to Windows 2000. There are four major suites of products to consider for this, each with their own strengths. Aelita Software's Controlled Migration Suite is the most robust product out there for "big" migrations jobs and is probably the one you want to consider first for something like this. Also worth looking into is NetIQ's Domain Migration Administrator, which is much easier to use and cheaper, but is not quite as flexible for larger migrations.
This was first published in May 2003