An alternator upgrade provides more current, preventing the battery from draining long-term. This is what you need if your battery goes dead after playing the stereo for a long time while the engine is running.

"Lights dimming" when the bass notes hit is a different problem, and is caused by the battery and alternator not keeping up to the transient demands of the system. For this kind of problem, a capacitor will do an excellent job of helping the problem by evening out the system demands. Increasing alternator output has little or no effect on this problem.

One "alternator" upgrade that can help is to improve the voltage regulator. The voltage regulator needs to be able to react to changes in demands on the electrical system, and most of them just aren't fast enough. The battery also has a reaction time, but it's a lot faster than the regulator, and except in extreme cases, can usually keep up. One of the primary reasons why an Optima battery is better for audio systems is that it has a faster reaction time.

It's important to correctly identify the problems that you're having, and provide the correct solution to it. If you really need an upgraded alternator, no size of capacitor will help. And if you really need a stiffening capacitor, no amount of extra alternator capacity will help, although a better voltage regulator and battery might.

I had an upgraded alternator, but now I'm running a stocker again, because it's good enough. If I keep my system cranked up all the time, it will drain the battery, but since I never really do that, the stock alternator is adequate. I have a .5F capacitor because I need it. Even with the upgraded alternator, (and voltage regulator) I still needed the cap.

I'm a firm believer in the correct solution for the problem.