Upgrading controllers to NT 5.0 will be quite demanding in terms of the hardware needed. How demanding is hard to determine because the NT 5.0 development process is still in its early stages. Although Microsoft usually cries foul when the press comments on beta speed, I'll venture this prediction: Because my experience in the past 10 years has taught me that the final products aren't much faster than their betas, I estimate that you'll probably need a +166MHz system with at least 64MB of RAM to run a bare-bones NT 5.0 system. Most people will likely start their systems more along the lines of a Pentium Pro with 128MB of RAM.
When you set up your enterprise NT 5.0 network, you'll need to describe the enterprise in two ways: its domains and its sites. Domains reflect your organizational layout; sites reflect the physical layout of your firm. Sites and domains are unconnected. You can have a site that contains domain controllers from any number of domains, and you can spread domains across sites.
A domain is a security boundaryin other words, an administrative partition. Unlike NT 3.x and 4.0 domains, NT 5.0 domains can encompass millions of users, so you can put your entire firm in a single domain. But many enterprises will, no doubt, opt for having many domains. Each domain will have a unique AD. Every domain controller in that domain will have a copy of the AD, which NT will replicate to ensure consistency among the copies.
But suppose the domain controllers in a domain are in different geographic regions and relatively slow WAN links connect them. Will NT 5.0 choke the WAN links with a lot of replication traffic? No, because NT 5.0 will use sites. Sites describe the physical layout of an NT network. The physical layout includes information on which machines have high-speed links and which have low-speed links. By default, relatively nearby domain controllers with higher-speed links will update each other every 5 minutes. Domain controllers with slower links will update each other less often. You can, however, change the update frequency.
Although you can control how often updates occur across WAN links, a nice feature would've been designing the network software to do some of that work instead. This feature would've been possible (sites provide NT 5.0 with the information it needs to use site-to-site links intelligently), but Microsoft missed the boat.