Hi:

I recently created a model of transaction processing across the Web middle-tier at a very large company, including interfaces to alot of mainframes.    What I did was use data flow diagrams, where each transaction was a data flow which I "flowed out" start to finish.    Important:  Since no to two transactions had the same flow path, it was critical to flow out  each transaction seperately. 

Data flow diagrams are ideal for starting at the context diagram level (a context diagram is a data flow diagram), and then breaking it down from there.

Tony