Do you have extra partitions available or the capability of adding another hard drive? If so, you can move your Exchange information stores from the primary partition using the Exchange Performance Optimizer. This might alleviate the space problems on your primary parttion. If you plan on migrating your domain from NT4 to Win2K (as opposed to just creating an entirely new domain, creating new user accounts, etc.), Win2K (and AD) MUST be installed on your NT4 PDC.
An alternative would be to install NT4 (as a BDC) on your new server, synchronize your domain, promote your new server to PDC, and then reinstall Win2K (and AD). This will take care of migration of your domain.
The next step will be to move SQL to the new server and upgrade. You need to follow Microsoft instructions for doing this (sorry, I don't have any experience with SQL).
If your definition of "rebuild" your present PDC is to wipe the drive before doing a clean installation of Win2K, you have some shufflingto do to maintain your Exhange organization. If you really want to do this, here is probably the easiest path to follow:
1. Install Exchange 5.5 on the new server as an additional server in your present Exchange site.
2. Migrate all mailboxes, connectors, settings, etc., to the new server.
3. Rebuild your old server, installing Win2k, and join it to the domain. Run dcpromo if you wish to turn it into an additional Domain Controller in your AD domain.
4. Reinstall Exchange 5.5 followingthe same steps as in 1 and 2 above.
5. Upgrade to Exchange 2000.
