A capacitor is used to take the AC ripple out of the top end of your positive and the bottom end of your negative. It doesn't help if you do not have the charging system to back it up like most people would assume, since it needs to be recharged as soon as it drains.
Basically if you were to hook up an oscilloscope to your charging system you would see something like this at the top of the positive "line"
__^__^__^__^__^__^__^__^__^
Due to the alternator actually creating a 12 volt AC current and using diodes to separate out the Direct current. A capacitor would just smooth that out.
--^--^--^--^--^--^--^ or possibly flatten it out all together. ____________________
Thats how it was explained to me, and I hope that helps you. Your best off getting a high powered alternator and running multiple batteries to negate the large current draw from the amps which in turn causes light to dim. The batteries act as a buffer and store the charge, if you can, placing one as close to the amps as possible is the ideal way of doing this, so the alternator is sending the charge to the battery and not having the current "sucked" out of it strait to the amp causing more wear and tear since the amplifier is not a constant draw but fluctuates.
If your going to run multiple batteries, I would use two of the same batteries, same models is best. If you have two different batteries, they will fight each other and discharge. You can put a shut off relay in between the two to keep that from happening, but why not spend that money on two of the same ones since those kits usually cost as much or more than a traditional battery.
Also if its going to be in cab with you, it HAS to be sealed. This is not optional since traditional batteries leak acid and noxious fumes while charging and discharging. One major benefit of sealed batteries is they can be mounted any way you want.
You may already know all this, but I'm just tossing that out there for anyone else browsing through.