What you were waiting since TMG 2010 was just released, now you can make your calculations to see which hardware you will need for your TMG Deployment:

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

To read more about this tool review the following post on ISA/TMG Team blog:

http://blogs.technet.com/isablog/archive/2010/02/11/forefront-tmg-2010-capacity-planning-tool-now-available.aspx