I always feel a little bad to tell my friends who buy outrageously beefy servers to run their ISA firewalls on. For example, a quadcore Xeon with 16GB of RAM to maximize speed for Web caching. 
Well, it sounds good on paper, but the fact is that there are hard coded limitations due to ISA being a 32bit application written to run on a 32bit platform.
For the details, check out this great article by Jonathan Barner on why the /3GB switch, PAE and AWE (Address Windowing Extensions) aren’t much help at pushing ISA past it’s 32bit heritage.
https://blogs.technet.com/isablog/archive/2009/04/07/isa-server-the-3gb-switch-pae-and-awe.aspx
The good news is that the TMG firewall is a native 64bit application and thus isn’t exposed to these limitations.
HTH,
Tom
Thomas W Shinder, M.D., MCSE
Sr. Consultant / Technical Writer
Prowess Consulting www.prowessconsulting.com
PROWESS CONSULTING | Microsoft Forefront Security Specialist
Email: [email protected]
MVP — Forefront Edge Security (ISA/TMG/IAG)