1) Perform a System State backup of the current machine -- when configuring the backup via the Microsoft System Backup tool, make sure that you specify that the system state backup goes to a single file.
2) Back up the entire server to tape.
3) Install the new server with Windows 2000.
4) Copy the System State restore file from the old server.
5) Take the computer off of the network - unplug the NIC cable.
6) Perform a restore of the System State making sure to indicate that it should overwrite existing files.
7) Reboot the server after the restore.
The system will undoubtably complain about new hardware, so you will have to have a Windows 2000 CD and any drivers you might need for SCSI and RAID Controllers handy. You will then need to spend some time cleaning up the driver database. The system should be operational.
I would definately do this twice! First time do it as outlined above and using the information in the article that I have supplied below. You can then work on the drivers and make sure you have them all and can get the machine working in a test scenario before you do it in production. Once your testing is done, reinstall the server with a fresh copy of Windows 2000 and start over. In production, you will want to make a backup of the current production DC, Copy the System State backup to the newly built server, and then SHUT DOWN THE PRODUCTION server while doing the restore to the new server. Since you did a proper test, you will already know all of the issues you will be facing with hardware drivers and will be prepared.
Here is a link to an article that contains more information: http://support.microsoft.com/default.aspx?scid=kb;en-us;263532&Product=win2000