I want to... no, need to, upgrade my sound system. It's already after market but it sounds terrible

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Anyway what are the size of the speakers and how many are there? I have a 4-door '92 318i. As well I want to put 1 sub in somewhere. As well I already have a Sony 10 cd changer but it doesn't play burnt cds so that definitelty needs an upgrade. I have a Sony XPlod deck, not sure of the model, I might keep this deck, i'll see the sound quality after I swap the speakers.
Please help me in making this shopping list for the next time I go to the car audio shop
Thanks ahead of time.