I recently added a subwoofer to my car. It's powered by a 300w JBL amp (and I also have 300w speakers). I currently have a 180amp alternator and a Die Hard platinum battery, which both handle my audio just fine, but I hear horror stories about subwoofers dimming lights and such on cars. I'm worried, that even if this isn't happening the high spikes that a subwoofer can draw is hard on me car, even if the alternator and battery can handle it. Would adding a capacitor help ease the load on my system.
Oh boy, oh boy. The never ending debate on Audio caps and their usefulness. I stumbled into this world a while ago, and based on practical experience with a cap, I will tell you that yes they will keep your headlights from dimming.
That being said, I'll tell you that I don't use my Cap anymore, not because of a different preference, but the amp it was connected to shorted out, and I needed room for a bigger amp, so the cap had to go.
The cap was connected to a 2000w peak amp, and it helped with dimming headlights, and also helped with muddy bass, all my bass was extremely stiff.
Now, I have a 2000w RMS amp, 4000w peak, and yes, if I turn the bass up high enough, the headlights will dim on super hard hits, but it's really not that bad. I have the same alternator as you, so I know I'm not getting the full 4000w, but it still makes me feel better :)
There are strong opinions on both sides of the fence. The people opposed to them say that they actually do more harm than good, because since your alternator now has to supply current to your battery and your capacitor, it's not the best solution.
The people for them say that they supply power quicker than your battery, in the event of a strong peak (hard bass hit). That way, your battery isn't drained as much, thus no dimming headlights.
For a 300w peak sub, I wouldn't bother with a cap. My cap was 20 Farads (yes, twenty. @ least that's what it said) and it was attached to a 2000w peak amp. I had a 700w peak amp connected to 2 12s once, I don't once recall the headlights on my '94 Grand AM dimming.
The people who designed your car probably didn't have subwoofers in mind when they built your vehicle. Big bass sucks up a lot of power, and most car electrical systems aren't equipped to deal with it. A capacitor acts as a buffer between your amps and your car's battery. You connect the cap inline on the power cable from your battery, as close to the amp location as possible. It stores up power from your battery, then releases it instantly to satisfy your amp's demand for the power needed to reproduce a big bass hit.
Have you ever noticed a big drop-off in performance after running your subs loud and hard for a minute or two? Or watched your headlights dim in time to the music while you're driving at night? A capacitor cures these problems by taking the brunt of those demand peaks away from your amp, so your amp sees a more consistent supply of power.
The rest of the article seems fairly complete, giving you a pretty good idea on your car audio system. As for the size of capacitor needed, Crutchfield gives the following from their FAQ page:
Q: What size cap should I get?
A: The rule of thumb is to put in 1 Farad of capacitance for every 1,000 watts RMS of total system power. But there is no electronic penalty for using larger value caps, and in fact, many see benefits with 2 or 3 Farads per 1,000 watts RMS. The larger the cap, the faster it gets ready for the amp's next big hit.
It appears capacitors can be from .5 Farad up to well over 100 Farads. If you are truly interested in what a farad represents, Wikipedia states:
One farad is the value of capacitance that produces a potential difference of one volt when it has been charged by one coulomb. A coulomb is equal to the amount of charge (electrons) produced by a current of one ampere flowing for one second. For example, the voltage across the two terminals of a 47 nF capacitor will increase linearly by 1 V when a current of 47 nA flows through it for 1 s.
So, overall, I don't know if you truly need a capacitor on your system, but from the sounds of it, it won't hurt a thing (but maybe your pocketbook).