Microsoft has released a capacity planning tool for TMG 2010

http://www.microsoft.com/downloads/details.aspx?FamilyID=01b2f7a5-8165-4ead-9693-994504f66449&displaylang=en

Overview

The hardware requirements for servers running Forefront TMG vary, and are dependent on a number of factors:
• The features you enable.
• The number of concurrent users.
• The maximum available wide area network (WAN) bandwidth.

The Forefront TMG 2010 Capacity Planning Tool, a Microsoft Excel spreadsheet with built-in workflow, allows you to calculate the WAN bandwidth and number of users a particular a hardware configuration will support, and can recommend server hardware based on the features, bandwidth and users your deployment needs to support.

For additional details, download the tool and see the “Instructions” tab.