Capacitors

What you're saying here is that you believe there's no more than 40 amps flowing at any time, right?  From what you have said that's probably true, especially using the more efficient class D mono amp for the sub.

If the headlights dim with the voltage at 14, you should look at the car's electrical wiring and find out why.  Better yet, you should do some better testing as your statement re: voltage sounds a little bit iffy.  Find out if there is a current problem with the lights in that car.  You should make it priority number 1 to start an upgrade of some of the OEM wires under the hood.  Upgrade the big 3 and then see if there are any problems.  Other than that, you're not a candidate for the addition of a cap for several reasons that you have stated:

• I don't have that serious of a system
• I don't listen to my bass that high that often
• nothing serious, and only at stops
• (OEM alt that came with the car)

The original alternator was most likely selected for the car because it was enough to power everything that the car came with.  Manufacturers don't overbuild and throw extras into the car "just in case".  But if you just want a cap for the sake of having it, sure.  BTW, there's a new sticky above that deals with cap issues.

It could be as simple as cleaning up and sanding or scraping some ground contacts in the engine bay.  Or just replacing the battery-to-chassis cable.  I think a lot of guys get the idea that a cap solved a minor dimming problem but it was just one of several things that was done to the car at about the same time.  Some guys want one just for the fact that they have the belief that it should be a part of a loud system (and it probably benefits also by shutting up all their friends who are constantly advising them to get a cap!).

Here is the thought I took with me after reading Rob's ground sticky:  part 4 is the big brother of the big 3.  It relates to the sometimes poor conductivity of a car's ground path because of the various ways that the factory attaches one part of a car's chassis to another part.  That is to say, if you just can't get a good enough path through the chassis for the current to flow back to the battery (after the big 3 upgrade), then running the ground wire directly back to the battery is the thing to do.

If that's not the fourth part you're referring to, I don't know.  But it's just basically good connectivity from any part of the electrical system, which includes the car's sheet metal chassis, to every other part.  Negative to negative, positive to positive...of course.

• Battery neg. to chassis - since amplifiers are grounded to the chassis the current flow must be allowed to flow as freely as the power wire current from B+.  OEM wiring here is smaller than most power wiring used in aftermarket systems.  You can leave the original wire intact.
• Alternator to battery pos.- gauge needs to be large enough for everything electrical in the car, including the add-on sound system.  Usually done by attaching a new large gauge wire directly from the alt to the battery +, leaving the OEM wire in place.
• Chassis to engine block - the alt is grounded directly to the engine block (bolts) so a good connection between that and the chassis metal will ensure good current flow.

When your car is showing signs of a power overload in the headlights, it is a visual sign that there is also an overload condition everywhere else.  You just can't see the results everywhere else.  And in your case, the OEM alt should be able to handle the added draw of your system (from what I gather).  It's all those secondary electrical components and the wiring between each one of them that has to be examined, cleaned or upgraded.  A fresh battery and clean posts is also a good start.