I would do a fresh install to a VM, add it to the domain as a DC with DNS,
migrate your FSMO roles and GC to the new VM, wait for replication then you
have a new 2003 DC in your VM, you could then upgrade your 2000 VM if you
wanted to have a second DC for redundancy.
In 2003 there is no Primary DC role so you add performance and redundancy if
you were to have a DC VM on both of your physical servers.
Good Luck!
Adam
>> Stay informed about: Advice on server 2000 to server 2003 migration or upgrade