It's important to make a distinction between domain structure design and site topology design. Sites reflect the location of user communities, whereas domains contain objects. A domain is mapped to a site by placing a replica of the domain within the site. A site doesn't contain a portion of a domain. It contains the entire domain.
Sites contain domain controllers are entire replicas of a domain.
If a GC is placed in a site, then the entire Active Directory becomes available to the site. From a different perspective, if a site doesn't contain a domain controller or a GC, it's essentially useless as no objects are available within the site. One major benefit of migrating to Windows 2000 is the ability to consolidate servers, including the total number of domains and domain controllers, to reduce the cost of hardware, troubleshooting and management. Planning for consolidation involves sizing domain controllers and GCs. During this process, you'll want to pay particular attention to the choice of server and storage hardware.