Caps actually put more strain on your already struggling alternator. Your alternator needs to supply everything with power and a cap just adds more it needs to supply. You should upgrade the alternator before getting a cap. If you hit a long bass note and your lights are dim thru the whole thing, you need a new alternator or need to have yours rewound for more power. If the lights go back to normal before the note is over, then the alternator just can't keep up with that initial jolt of power, that's when you need a cap.
Hope this helps, if not, shame on me.//content.invisioncic.com/y282845/emoticons/nono.gif.eca61d170185779e0921b0faa9704973.gif