Exchange 5.5 to Exchange 2000 migration

I’m looking for some suggestions for the best way to upgrade my existing environment to Exchange 2000. I am currently running a WinNT 4.0 (SP6)server with Exchange Server 5.5 installed(strictly email and user space) and a Win 2000 server which hosts our business software (main app). The NT 4 is a domain server and the Win 2000 is setup just as a workgroup server (Active Directory is not installed). My plan is to purchase a new server to host the Exchange 2000. Should I setup this new serveras the domain controller in the active directory and just add the existing Win 2000 as a member server or vice versa? Or could I just leave the existing Win 2000 as a workgroup server?