Microsoft published a nice document (they call them Notes, I am pretty sure this has no relation to Lotus at all ) on their move to 64 bit servers and Exchange 2007 being 64bit.  For customers still feeling the pain of needing 64bit server for Exchange 2007 this document may help to allay the concerns.

Description:

This document discusses the most important issues that forced Microsoft IT to stay with 4,000 users and 200-MB quotas on the 32-bit platform. It also describes the important trends and emerging compliance necessities that compel Microsoft IT to increase service levels now, such as by supporting larger mailboxes with 2-gigabyte (GB) quotas. In addition, this paper discusses the server designs, including microprocessor choices, random access memory (RAM), and storage configuration, and the backup strategies that Microsoft IT defined for the rollout of Exchange Server 2007.

Download here:

http://www.microsoft.com/downloads/details.aspx?familyid=f31e7541-f63a-4b7d-b8d2-3794c4dc3329&displaylang=en