I would be out of my depth to advise on the code changing and I am not familiar with Mambo
Here is a link which will explain SS! in more detail.
http://bontragerconnection.com/library/The_Basics_of_SSI.shtml
I am only semi technical so I love SSI as i can specify whole sections or just part as an SSI file so when I want to do updates across 100+ pages I just change one file.