A very useful tool for BlackBerry administrators and support staff is RIM's BlackBerry Enterprise Server Compatibility. Just head over to this link, pick your email system (Microsoft Exchange, Lotus Domino, or Novell GroupWise) and you will get a PDF with a very detailed matrix.

This matrix details operating systems, versions of database servers, version of instant messaging infrastructures, etc. Keeping this matrix handy will certainly cut down on the number of questions that remain unanswered when it is time to upgrade a component of your network.