A question that has come up a few times in various Microsoft
Exchange newsgroups concerns Exchange’s support for 64-bit systems. The simple
answer: While Exchange can run just fine on a CPU that has 64-bit extensions,
the underlying OS must be of the 32-bit variety. This is true for the Exchange
server itself and for any system on which you install the Exchange System

This is not to say that you need to forego 64-bit computing
altogether, however. Keep in mind that Exchange is fairly tightly coupled with
Active Directory and, therefore, with Windows domain controllers. However,
since Windows is the Active Directory provider, non-Exchange systems, including
domain controllers, can be 64-bit and Exchange will work with them just fine.

The next version of Exchange, Exchange 12 (also being called
E12), is slated to fully support both 32-bit and 64-bit systems and support
both AMD and Intel extensions as well as dual core technology.

Besides Exchange itself, if you use other products, such as
fax servers, and they run on separate servers, be careful before taking the
64-bit OS plunge. Many device drivers and a lot of software continue to be
unable to support these fledgling platforms. By the time Exchange 12 arrives, 64-bit
will be much more common.