There is a new technical document regarding the impact of installing BlackBerry Enterprise Server (BES)5.0 on the performance of a Microsoft Exchange infrastructure.
The impact can be quite significative, but there’s nothing as reading the whole document.
You should configure the Microsoft® Exchange messaging server with sufficient processor capacity and disk throughput capacity to manage the additional resource load caused by adding BlackBerry device users to your organization's environment.
You cannot apply simple multipliers to the messaging server resources because other applications on the messaging servers, such as antivirus software, backup processes, auditing software, and archiving software, might impact the load statistics.
It is a best practice to use Microsoft Exchange 2007 or later. You should configure the Microsoft Exchange server to have access to as much memory as possible. Microsoft Exchange benefits greatly from having access to sufficient memory to cache much of its data. This greatly reduces disk traffic and the impact of the BlackBerry Enterprise Server on disk resources.