I have been thinking about caps but every body gives them such a bad rep so I thought I skip right to batteries. I was also wondering why do battery companies like kinetec and xs market there batteries as for audio systems when they know that 12 volts wont cut it.

 

A Batcap would help but it's like putting a bandaid on a cut that needs stitches.. Just doesn't quite close that gap.

But it may alleviate the issue enough to sate you.

 

don't waste your money on a capacitor
do the BIG3 first, and see if your dimming goes away. if it doesn't, the money for a cap would be better spent on a higher amperage alternator IMO.

with 500wRMS, a second battery is really only needed if you spent alot of time listening to your stereo, with the car turned off.

 

The big three is the best/cheapest solution that has been said in this thread. This will help keep your voltage at 14.4 without having to buy a High Output alt. If you buy a high output alt, you will have to do the big three anyways (at least the alt -> battery)