WOW – where does one start? It’s like going from a vw bus to a BMW, where do you begin to describe the differences? Exchange 2003 you could run on one box, and you can do that with Exchange 2007, but if you have a medium to large organization you will want to break the roles out onto different servers. There are 4 server roles in Exchange 2007, only three of which can run on a single server, Client Access Role, Transport Server role and Mailbox role. The Edge Transport role must be on a different server in the DMZ. Exchange 2007 only runs on 64 bit servers. It uses memory differently so you can add much more RAM to the server than 4GB and there is a lot of planning involved to decide the best memory configuration for your server usage. There are many things to consider in the planning of an Exchange 2007 install – note this cannot be an “upgrade”, it must be a new install. You can use cluster continuous replication or local continuous replication or standby continuous replication configurations. Depending on whether you choose one of those options will decide the size of your mailbox stores and individual mailbox size limits, and you DO need to set those limits! Here is a basic comparison chart, but it is not comprehensive to design. Public folders are de-emphasized but still supported. You can integrate voicemail into Exchange 2007. Here is another good article for you. Here is a good article for designing an exchange 2007 install. There is a LOT of research and planning involved if you are going to move forward with Exchange 2007, however I feel it is well worth the effort and the change. Much better performance with Exchange 2007 in my opinion! But there is a LOT to learn! I suggest you take a class if at all possible.