Exchange Server 2007 Question:

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For the best performance, when an Active Directory organization contains more than 20,000 objects, you should upgrade to 64-bit. Upgrading servers that run Active Directory domain controllers and the global catalog to 64-bit improves the overall performance and scalability of your Exchange Server 2007 environment. However, 32-bit domain controllers are still supported.
Lookup and response times between the Exchange 2007 categorizer and the Active Directory directory service will improve with the use of 64-bit. The size of the Extensible Storage Engine (ESE) database that holds Active Directory can frequently be larger than 3.0 gigabytes (GB). This prevents caching of the contents of the whole database, and therefore increases lookup and response times. By using 64-bit, the available RAM for caching can be increased beyond 4.0 GB. This is large enough to cache the whole ESE database, even for large Active Directory organizations, and will improve Exchange 2007 lookup and response times.