Second battery?
In my opinion Capacitors are overrated. If you're doing a big time stereo, you might as well get a high output alternator to go with it.
If you're running your stereo with the ignition off, then a second battery is a must, but a capacitor still doesnt do anything for you, other than drain the battery faster.
I had dual 270xp DC alternators in my Yukon with two Odyssey 100wh batteries. I never had a problem pushing 2000 watts for a couple hours. With no Capacitor.
to me, most Capacitors are for looks. It completes the whole amps, subs, crossovers, caps look that some people are looking for. I'd rather hide everything, and go big alternator to keep things running without dimming lights.
The cap is there to recharge for the next bass hit, so you dont have voltage drop, but with a sufficient alternator, you'll never notice a voltage drop anyway, and you'll have piece of mind knowing you can run other electronics (inverter, lights) in any situation, without worrying about brownouts.
No offense, just giving my opinion!