well ive had about 1400 watts going for like 8 months now and the lights barely dim when it is pounding, not bad for stock.. Dont really need a cap or new battery til your around 1500, im kinda pushin my luck right now..
You should have a 130amp alternator. That is plenty of power for your amp. A cap will still make a difference though. It will give you a bit more punch that you can't get by just a stock electrical system alone.