Its not usually the amp rating of the alt. that is the problem its the fact that your subs and amp draw the power off and on very quickly. if your going to install a sub. If your just looking to give a little boost to the main speakers then you probably wont need to do anything. a capacitor would be the first thing to look at if your putting in a sub and/or large amplifier. that fast on and off cycling stress a sub puts on an alternator will usually blow out the diodes so a capacitor will help stop that by acting like a battery so it drains power from the capacitor first.