some car companies are saying that their car has a 6-cd changer with mp3/wma playback capability or watever...what exactly is that??? from what i understand, there is a blank cd out there that holds up to like 4.2 gb of music or something like that...am i correct??? and if i am correct, can somebody please tell me which format of blank media that is that i need to buy to hold all that music and play it in my car?? thanks