I got this information about how the kernel memory is managed in an Exchange 2003 server. Here you'll find a series of three articles, which i'll be posting as soon as I get them. Following there is an introduction to the complete series which you could access from the links at the bottom. Have a long read on it, check out how 32-bits works and be prepared for the next generation of Exchange. ;)

Kernel memory resource bottlenecks can drastically limit Exchange 2003 scalability. Kernel resource usage may vary greatly from one Exchange server to another. A hardware platform that can support 4000 heavy users in one organization may be limited to half that number in a different organization because of kernel memory exhaustion.
Large increases in kernel memory consumption can be triggered by changes that few would anticipate as problematic. This could cause sudden and widespread Exchange server outages throughout an organization.

Article 1: This article applies specifically to Exchange Server 2003 running on Windows Server 2003. However, much of the information presented here applies generally to application scalability on a 32-bit computing architecture. 

Article 2: In this article Windows 2003 kernel memory issues and Exchange 2003 will be reviewed. This article discusses important hardware issues that may affect Windows kernel memory and cause Exchange instability.