A cap is like a band aid, it's just covers the problem. If your running equipment that draws high Amps, maybe invest in a second battery in the boot (optima etc...). Also do the BIG 3 upgrade, By earthing the battery and chassis with 0 guage wire and adding 0 gauge wire from the alt to the postive on the battery. This should ensure you won't drop below the 12V mark and also eliminate headlight dimming al together.

Also look outside the square, with brands like DD,Morel,hertz,sound digital, setetsom, MM, atomic. All these brands make really top notch amps and subs. If your gonna buy a system. Spend the money now and then you wont feel the need to upgrade when you feel your cheap set up isn't loud enough.

We have all been there before. Iv'e spent close to what a new getz cost on my system and im not regretting anything.

Also make sure you use the correct guage wire in your install and don't cut corners cause that is when fires start and you can void your insurence