Microsoft's Active Directory (AD) Sizer helps you determine the domain controller hardware that your AD design will require. The tool is available at http://www.microsoft.com/windows2000/library/planning/activedirectory/adsizer.asp.
You specify the number of objects you expect to host in your AD and the number of sites. AD Sizer also asks for information such as the estimated number of domain logons per second and the frequency of password changes. So, before running the tool, capture some of this data from the Security event logs of your current Windows NT 4.0 authentication-domain PDCs. The NT 4.0 traffic should be fairly representative of the traffic you'll see in your Windows 2000 AD infrastructure.
Using the data you enter, AD Sizer calculates the amount of disk space and RAM you need on your domain controllers and how much traffic you can expect to see going through each of your sites. Figure A shows sample AD Sizer output. A domain of 50,000 users and 100,000 computers will generate an ntds.dit file of 2.1GB. For this example, I specified only one site. I could also specify multiple sites and distribute the 50,000 users between the sites, and AD Sizer would tell me how much network traffic I could expect between the domain controllers in each site.