Establishing your current domain structure under the new environment should test the limits of your server hardware. The new directory system is touted as being more scalable than its predecessor, but your domain structure may be so complex that it pushes the new system to its limit. If this is the case, consider remodeling your domain structure on paper and testing it against the new server. In particular, you should focus on the replication issues that will arise when you spread servers around the network to replace your existing Windows NT 4.0 primary and back-up domain controllers. If you're using commercial software products, you should make sure they've been tested in a Windows 2000 environment.
Large servers are difficult to handle in a pilot program.
They're expensive to duplicate and a small model may not reflect the true behavior of a larger system. Since servers don't scale linearly and the very nature of the OS is different, what you would get in a model is, at best, a crude approximation. Storage servers, for example, will behave differently with the new filesystem. Setting up a smaller version of a RAID system may not reflect the proper performance behavior, though it will highlight changes to the management structure with which you may need to familiarize yourself. If you're using commercial software products, you should make sure they've been tested in a Windows 2000 environment.