Is your DHCP and DNS running in on your DC? Are you a mixed protocol environment IE: IPX and TCP/IP?
If so your DC goes down then your DNS and DHCP would go down as well then they would be very limited to Network resources. All it really takes is a workstation loaded with Server 2003 to be a backup. You'll pay less for the license and workstation then you would for the down time and loss of productive time.
If they don't budge then I would ask you if you have any experence with backup and restoring AD? It sucks and almost never works. I would bring up a second box on VM to cover my own ass, and then accidently have a power cable come out of my DC for a couple hours to prove my point. Most managers don't understand what you are talking about until it happens then they want to blame your for not predicting the future correctly. I had to fight for the redundecy that we have in our environment. Now I don't have to schedule down time because I just reboot and the other servers take over for the one that is rebooting.
Good Luck
LZ-KID
The fluffy midgets in my head tell me to format everything and hope the end user knows how to backup their data.