Whether one is designing a new network or making changes to an existing one, planning is the essential, nonglamorous, before-everything-else component. Few enjoy and many actively dislike this task, but it's important to understand that every minute, every hour spent planning will later repay you (or your heirs and assigns) a hundredfold.
Sometimes it's difficult to appreciate the benefits of careful planning—until you've had the misfortune of supporting a poorly considered installation. Windows 2000 is particularly unforgiving of an offhand approach to planning. As has been mentioned before, you can add Windows 2000 Server or Windows 2000 Professional to a Windows NT 4 network. However, unless you plan on eventually using Active Directory and other Windows 2000-specific tools, the benefits of just plugging in a couple of Windows 2000 machines are minimal. And in all cases, designing directories—not to mention applying new concepts such as forests and trees—requires considerable thought and the ability to project current experience into the future. Therefore, the wise systems person takes a longer perspective.