I agree with whats been said. It is always hard to tell what the amplifier draw will be for any song or amplifier setup. Usually if your running your amplifiers bridged (usually 1 or 2 ohms) the efficiency is going to be quite a bit less then what is stated. Anyhow, for that kinda RMS power, I would upgrade the alternator and not get a second battery. A second battery will do nothing for your cars electrical system except let your stereo play when the batteries arnt being charged. The stock alternator doesn't produce the kinda amperage your system requires, so i would upgrade the alternator.
Good Luck.
