system speed

I am a junior DBA and have been assigned the task of analyzing our informix engine and making recomendations on how to improve performance. Obviously there are far too many variables to post in one posting. Would anyone out there care to offer some assistance in this matter. I am very new to to informix and would appreciate any help. The instance is running on a SCO box with four intel 200 p-pro's. 700 megs of ram, over 100 users on a busy day. I think at this point there will not be any physical changes made unless a dramatic improvement could be acheived for minimal expense. We really want to tune the engine.