I want to set up a front-end/back-end architecture for an Exchange 2000 Server deployment. Do I have to use Exchange 2000 Enterprise Server?

Exchange 2000 Server, Standard Edition can't be a front-end server, but it can be a back-end server. The front-end server does nothing more than act as a proxy for whatever protocols you're using and perform some directory lookups. However, bear in mind that using Standard Edition as a back end limits you to one storage group (SG), two databases (one public, one private), and 16GB of database data.