Best way to build a replacement server

I am running a Windows 2000 network which consists of three Windows 2000 Servers. Two DC’s and a stand alone ISA Server. One of my servers is an Exchange 2000 server, and I’d like to build another server which I will have ‘on the shelf’ in case of a failure of my current Exchange Server.
I am a little unsure of the proper method to accomplish this. My concerns are mostly with Active Directory, and SIDs and such.
What would be the BEST way to have a server ready to in place of a current server, but have the transition transparent to the user. Would a Backup/Restore of AD from one machine to another do the trick?
Any suggestions are greatly appreciated.