Microsoft has released the Active Directory Sizer. This is available for free download and will be part of the W2K resource kits in the future. The commandline tool is designed to let you estimate the hardware required for deploying Active Directory based on your organization's profile, domain information, and site topology. Based on the information you input, Sizer estimates the number of the following:

• Domain controllers per domain per site needed
• Global Catalog servers per domain per site needed
• CPUs per machine and type of CPU needed
• Disks needed for Active Directory data storage
• Amount of memory required
• Network bandwidth utilization
• Domain database size
• Global catalog database size
• Intersite replication bandwidth required

Its modelling is only as good as the data you enter. Do due diligence on the data entry and you should get out decent estimates. There are many options depending on your organization structure and application needs. if you use this tool and find it valuable.