If actually planning on retiring the 2000 server;
I would attach the new 2008 server to the existing domain, dcpromo it leaving the domain/forest functional levels where they're at, then transfer all FSMO roles and other services(file server, DHCP, DNS etc) to the new server, once you retire/demote the 2000 server, you can raise the domain/forest functional levels on the now prominent 2008 server.
As long as that 2000 server is a domain controller, you're stuck at the 2000 levels.
http://technet.microsoft.com/en-us/l...09(WS.10).aspx http://www.microsoft.com/windowsserv...rade-2000.aspx