If your total system power is 1200 watts RMS or more, you will need to:
1. Upgrade the "Big 3" - that is to replace (or add to) the power wire between the battery and alternator, the ground from the battery to the chassis and the ground strap from the engine/tranny to the chassis.
2. Upgrade to a yellow top battery
3. Upgrade the alternator to a high output alternator
Failure to do this will result in the early death of the alternator as it will eventually fail.
Here is a guide that will calculate what size alternator and power wires you need http://www.box.net/shared/uchv4dbk88
You will need Microsoft Excel. If you don't have Excel, use this link http://www.box.net/shared/nkkw1dhk4g
A second battery is never a good idea as it's an added strain on the alternator to charge two batteries AND supply power for the car. The ONLY time a second battery is useful is if the audio system is to be used with the car off.
A capacitor is only good so far as the audio system isn't trying to pull too much from the electrical system. A cap doesn't provide more power, it's designed to 'stiffen' the voltage to the amp, nothing else. If the current isn't there, a cap won't help.
The alternator must have at least 20% more amperage power than the entire vehicle and sound system combined for a cap to be of benefit which is ironically the same requirements for an amp to be efficient.
See my site for more info http://spkrbox1.spaces.live.com
