I am being asked this almost every day. "Is Exchange 2007 ONLY 64-bit?" Well, YES and no. There is a 32-bit version that is to be used ONLY for testing, evaluation, labs, classrooms, demostrations, etc... It is NOT very well optimized and is NOT supported in production. I don't know how it will be distributed once Exchange 2007 RTMs, though.
Any server that will house production users or be part of your production organization (mailbox, hub transport, client access, or unified messaging servers) MUST be 64-bit servers (x64 architecture, not IA-64) running Windows 2003 64-bit. Read the Microsoft Exchange 2007 System Requirements
In place ugprades are NOT supported, so you will have to have at least one spare piece of hardware to start doing your migration. Most any server that you have purchased over the last year or two should have the x64 extensions,
What will the 64-bit architecture "buy" you? My opinion:
If you run a single Exchange 2000/2003 with a few hundred mailboxes and have no high-availablity features in play (such as clustering or disk replication), then probably not much. The 64-bit move for Exchange 2007 means "more RAM". That translates in to better performance, more efficient disk I/O (which is where larger servers have problems), and the ability to add more features that require improved I/O. Exchange 2003 is quite simply maxed out as far as what it can do; it is constrained by the amount of RAM it can access and therefore has to "go to disk" too often on a busier server.
With more RAM available to Exchange and the database engine, I/O is better optimized, and new features such as continuous replication (local continuous replication (LCR) and clustered continuous replication (CCR)) and per user 'safe sender'/ 'blocked sender' lists can be included.
Is Exchange 2007 better? Heck yes! I have a couple of customers that are already budgeting for migrations to not only Exchange 2007, but Office 2007 in the next year (provided there are no nasty show-stoppers in the RTM version.) 64-bit is a VERY small price to pay for some of these new features and it allows Exchange to move in to the 21rst century with respect to CPUs.