Yes I understand Glenn but it's not really about transactions per minute more so it is how much data per second it can handle. Generally speaking the travel agency is probably mostly doing SQL query transactions which are very small with respect to data size and a bit of graphics, etc.

Wikipedia is ranked #6 in the World for traffic and their traffic combined over the year averages just above 4Gbps with peaks just under 8Gbps. That amount of traffic would would be like a DDoS to TMG and ISA Server is not a deep packet analysis server either. Cisco does provide a dedicated device that they claim can manage 4Gbps of analysis in a single device. Passing 4Gbps is fast let alone actually analyzing it and routing it based on the analysis takes a lot of computing power.

I admit this is certainly not my area of expertise but basic research seems to show these dedicated boxes way out perform a normal server for intrusion detection and prevention.