You will need neither a cap or another battery. The cap you have your eye on is too large for your system anyway. The only thing that a cap is going to do is stiffen the voltage to your stereo system, that is all nothing more. The 2nd battery (like a cap) will just put an add strain on your electrical system in your car. You have no need for either, so use that money on something else. I am running 3200 watts rms to my sub and I have no need for a cap or an additional battery. You will be fine. If you do experience your head lights dimming, do the big 3 upgrade and that alone will cure that problem. Battery negative to chassis, battery positive to alternator, engine block to chassis ground.
