well the cap really wont help anything.. ...what kind of amps do you have.. ? a sencond battery would help and or a bigger alternator. check out this websight their is a ton of really good information here.. you might wanta do something called the big 3 they talk about it here, check it out.: hope it helps.
http://www.caraudio.com/forum/forumdisplay.php?f=13