is it possible to use either DiRT or COBRAS to restore to a new server with windows 2003 versus windows 2000 on previous server?
Yes, neither of them look at the OS for a restore (or backup for that matter)
DIRT just requires the same unity version
Actually if you review the OS upgrade guide for Unity it will tell you to rebuild from scratch using 2003 and to reinstall unity and then do a DIRT restore.
if this helps, please rate