Without knowing your real scenario and all its capabilities, my advice is:
Join the W2k8 server to the existent W2k3 domain. You’ll need to extend the existent AD schema to support w2k8 servers. Please run:
(put the w2k8 DVD in the w2k3 DC, change to ADPREP folder and run)
Only after these step you’ll be able to join w2k8 to the domain.
After joined the domain, run DCPROMO to promot this new server to a new domain controller on an existing domain.
Once Active Directory is installed then you’ll need to make the new machine a Global Catalog server.
For this, go to Administrative Tools, Active Directory Sites and Services, Expand, Sites, Default first site and Servers. Right click on the new server, select properties and put a check in the ”Global Catalog” checkbox. (Global catalog is essential for logon as it needs to be queried to establish Universal Group Membership)
If you want to transfer the FSMO roles to this new server, like if you want to completely disable/replace the old server, please follow this guide.