Okay, before you go replacing parts you need to figure out what parts are actually causing the fault. By "Main Speaker" i am assuming you mean the "Earpiece Receiver" that you listen through for incoming/outgoing calls? If that part is working and the digitiser (screen) is not faulty/intermittent/damaged then i don't think you will need to replace it.
Both the speaker phone and ringer come out of the same part (speaker located bottom left of iPhone), and if they are faulty then you will just need to replace the Dock Connector Assembly (includes a new Dock Connector, Speaker and Microphone). Here is a link to the part, here is a link to the repair manual.
Hope this helps! :)