The recommendations outlined here are Research in Motion recommendations:  

Minimum required hardware and configuration:

• For 500 users: Intel® Pentium® IV processor (2GHz or greater), 1.5 GB RAM, MSDE 2000 on the
  same computer
  
• For 1000 users: Intel Pentium IV dual processor (2GHz or greater), 3GB RAM, MSDE 2000 on the
  same computer
  
• For 2000 users: Intel Pentium IV dual processor (2.8GHz or greater), 3GB RAM, MSDE 2000 on
  the same computer

Note: For systems running over 1000 users, the recommendation is to deploy Microsoft® SQL on a
separate server. This reduces the memory requirement on the BlackBerry® Enterprise Server.

Minimum Software Requirements:

• Windows® 2000 Server (Server or Advanced Server Editions) or Windows Server® 2003
  
• Installed on IBM® Lotus® Domino® Server 5.03 or later (5.0.12 or later recommended), or IBM
  Lotus Domino Server 6.0.1 CF2 or later, or IBM Lotus Domino Server 6.5 or later
  Connects to Lotus Domino mail server 5.03 or later (5.0.12 or later recommended)
  
• Microsoft Internet Explorer® version 6.0 or later

Note: Certain features require a minimum version of BlackBerry software.