When to use different Active Directory Models
Windows 2000 makes it possible to configure the enterprise in a great number of ways. Since no two enterprises are identical, successful deployment is based on an understanding of how to employ Active Directory's flexibility, scalability and granularity. This article explores the relative advances of different domain models, the use of organizational units and basic site considerations.

http://www.windowsadvantage.com/trenches/03-13-00_admodels.asp