the only good capacitors really are, is to act as a digital voltage meter. id upgrade the battery before i got a capacitor, cant tell ya too much info without knowing the brand or specs of the amp.
capacitors take the load off your alternator, unless you have some super powerful alternator (if you haven't put an aftermarket one in, chances are you don't) it's a good idea to get one, it will do nothing but help and they are pretty cheap these days.
