One of the reasons to move to 64Bit in Exchange 12 is for more memory space. We are reaching a limit in sizing for a single server because of memory exhaustion of the 4GB limit in the 32bit application/os. In Exchange 2003 we utilize 1 GB for Operating System and 3 GB for Exchange 2003 operations. That being said the size or number of client access tokens can be the limiting factor in the number of clients that a server that is running Microsoft Exchange Server can support.
Each access token requires some Microsoft Windows kernel memory. The amount varies depending on several factors. Group membership is one of the most important factors. The size of the token increases in direct proportion to the number of group memberships.
There is a hotfix that is available today through Premier support that can increase performance on servers with high connection rates by up to 1/3.
To find out if you need this hotfix I would recommend reviewing this article: http://support.microsoft.com/kb/912376.
More info from the Exchange team: http://blogs.technet.com/exchange/archive/2006/02/27/420684.aspx
Hotfix info: http://support.microsoft.com/kb/912480