i think you need at least 50-60 amps for the car its self. good rule of thumb is to leave the stock amperage towards the car and add whatever amperage you will pull with your accesories to it and get a HO alternator of that size.
you'll just have to see if your lights dim to tell if you are pulling too much. obviously can't pull all 80 amps for a sound system though.
500 watts is about 40 amps for reference (watts/12.6=amps)