Why 64-bit?

Of all the new requirements for the latest version of Microsoft Exchange (2007), the requirement for the systems to run on 64-bit hardware and Windows has been met with the most confusion and in some cases contempt.  Granted, Exchange 2007 will install on x86 machines, but that configuration is strictly for testing, and is not supported at all for use in a production messaging environment. The reasons behind the move to the new Instruction Set are many, but the big ones are:

1 - It's faster.  Hands down, an x64 server running on the appropriate hardware will out-perform a 32-bit server every time.  This is due to both the architecture of the server being able to handle more I/O operations per second, and just due to the fact that you'll usually have faster hardware when you upgrade the physical machine to an x64 architecture.

2 - You can push it farther.  Windows 2003 x86 editions have much lower limits in terms of the amount of RAM you can put into the box.  For Exchange, this is a devastating limitation.  Even with the boot switches which allow for extended memory utilization, Exchange 2003 and 2007 (x86) can only use about 3GB of RAM at their maximum, though the Windows system may be able to address more than that number overall.  64-bit Exchange 2007, conversely, has a recommendation of 2GB baseline RAM with another 2-5MB for each Mailbox, meaning that 2000 mailboxes would have you looking at 6GB RAM, and more likely much more than that.  According to the Microsoft Exchange Team Blog, you should be aiming at 32GB per Mailbox Role server, something simply impossible on a 32-bit architecture.

3 - You get more mailboxes with less hardware.  32-bit Exchange servers don't have a fixed limit on the number of mailboxes you can put on a single server, but many Enterprise users have found a logical limit of around 2000-3000 users per server, if all the hardware is maximized out.  The reason for this is that 32-bit systems have a fixed limit of a little over 500MB of Kernel Mode memory, less than that if you use the /3GB switch at boot.  Each Outlook connection uses a small amount of this memory, so eventually you end up with Kernel Exhaustion, and new connections either get delayed (temporarily freezing Outlook)or denied (Outlook errors).  64-bit Windows has up to a whopping (at least by today's standards) 128GB of memory.  Though it will be limited to 40% of total memory on the box, with the right configuration you could have thousands of Outlook connections on x64 and never miss a beat.

In the end, the move to Windows 2003 or 2008 x64 and possible new hardware for your Exchange 2007 server will benefit you and the end users in your organization.  Especially in these days of having to do a lot more with a lot less, biting the bullet and investing in the new hardware platform will allow you to extend your investment much farther than you were able to on x86 architecture.  That means fewer hardware upgrades spread over a longer period of time, and fewer physical machines required to support the messaging systems you need.