Back when I was supporting a large enterprise Active Directory environment, I was tasked with upgrading our Domain environment from Windows 2000 to Windows Server 2003. The project was made much easier with the great documentation from the AD team published on TechNet. That same checklist based, simple documentation is available for Windows Server 2008. First you will want to get some background information covered in this document, then spend some time reading the detailed information on the upgrade itself.
At a high level, moving from Windows Server 2003 consists of:
If you are planning to upgrade Windows 2000 AD domains to Windows Server 2008, you must perform and in-place upgrade of all existing Windows 2000 domain controllers in the forest to Windows Server 2003 (supported upgrade path documentation). Then you can perform a second in-place upgrade of those DC's to Windows Server 2008. There also are a couple of other smaller steps you need to be aware of in a Windows 2000 to Windows 2008 domain upgrade, and the documentation covers all of those details.
I suggest you take some time to familiarize yourself with the documentation and utilize the checklists provided to make your upgrade as easy as possible.
Additional information about upgrading Active Directory domains to Windows Server 2008.