I'm with VPUPPY on this one. A battery is more or less a big capacitor and you'll get a LOT more capacitance by upgrading your battery than adding a big capacitor IMHO. Note: this is coming from a guy that represents a company who manufactures capacitors.

All you are really trying to accomplish is having stored energy at the ready for those times when the amplifiers call for it. I believe that a good quality battery and/or a larger alternator would be more effective and in my mind less of a bandaid than adding a capacitor.

Todd