i think a second battery would help more then a cap. at least for a show car.
if your car has a well built electrical system, (i.e. hi output alternator, a good battery, upgraded wires) you wont need a cap or a second battery for that matter. my alternator is about to take a **** on me so i'll soon be ordering a nice high output one and i'll then ditch my cap and even get bigger amps
hey dave- next time i see you i wanna give you a quick rundown on what i plan on changing in my system and see what you think. i'm kinda iffy about it and i'm not sure how it will do. [/hijack]