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 Manager.
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.