Its a small network that had just one domain controller with exchange on it, I recomended adding another domain controller and the hope was that I could run exchange on this too, the Idea being that if either server went down we would still be fully operational once the client machines had been redirected to whichever exchange server was still running. the Ideal would be that the two servers would be mirrored and the exchange mailboxes would then be identical on both, I have heard that you can get software that allows you to do this.
The only way I could see that was similar would be to create a replica of the public folders on the second exchange box, set the replication up, Then if I lost the main server I would only have to get the mailboxes onto the standby server.
just thought there may be a better method that I hadn't thought of.
Also I have had no problems running exchange 2000 or 5.5 on domain controllers