ISA Server 2006 Capacity Planner
If you haven’t used ISA Server 2006 Capacity Planner you might want to check it out. It’s an online tool that lets you plan secure publishing, branch office gateway, and Web access protection.The tool recommends how many CPUs, amount of disk space and amount of memory that will be suitable for you based on the questions that you answer. While you can argue that the tool is not really exact science but it is a great place to start if you are trying to figure out what kind of hardware you will need to configure and ISA Server in your environment.
You can check out ISA Server Capacity Planner here.