Can I intermix servers running Exchange 2000 Enterprise Server and Exchange 2000 Server in the same Exchange organization?

Absolutely—you won't find any interoperability restrictions when you intermix these two Exchange editions. The basic functionality is the same, and they interchange messages seamlessly.

For most people, the biggest difference between these two editions is the size of the stores. Exchange 2000 supports a single storage group with one private and one public store; these stores are limited to 16GB of data. Enterprise Server removes these limits. As a bonus, it supports clustering, front-end and back-end topologies, and Exchange 2000 Conferencing Server.