If you still believe you can migrate to Windows 2000 and Active Directory with only minimal planning, I'd suggest you read the document "Active Directory and Exchange: Dependencies and Requirements" at the Microsoft Exchange Web site.
Active Directory was based, in part, on the Exchange Address Book (from Exchange 5), coupled with NT's domain system. Exchange 2000 uses Active Directory as its address book and, therefore, requires Win 2000 servers. Trying to upgrade to Exchange 2000 before, during or after an upgrade to Win 2000 can leave you gasping for air as you try to figure out which accounts are active, which are dormant, which reside locally and which are in a different forest.
Microsoft’s document is thorough, outlining requirements and dependencies for sites new to Exchange 2000 and for those upgrading from Exchange 5.5. Additionally, it explains how the difference between new Win 2000 sites and those upgrading from NT 4 affects requirements and dependencies.
Even so, the document doesn't describe how you should upgrade from a different network operating system or a different mail system. That's probably good because I was becoming confused about the order in which I should do the upgrade/installation - and I'm supposedly familiar with Win 2000 and Active Directory.
Read the document, though, and then rethink your plan for rolling out Win 2000 servers.
This story, "Upgrading Exchange " was originally published by Network World.