capacitor vs. added battery

Get a bigger alt.

Battery and caps still have to get charged. The Alt does that. Have a low output alt and everything suffers.

Yes, the alternator charges them... but in order to charge them, they must first be discharged.  They are not loads, in fully charged form, essentially they are simply available sources, should the voltage level drop below each of their threshholds.

Secondly, if the current demand of the alternator were momentarily exceeded - say, a really big bass note - so that the alternator couldn't supply all the current for that moment - all that would happen is the battery would supply the additional current needed, however much that was.

And when that moment in time passed, the battery would recharge that small amount of power drained, in about the same time period.

A capacitor is lighting fast, to both charge and discharge, and it suffers no damage or wear from doing so, unlike the battery.

Bear in mind, on average, even high powered systems only draw about 20-30a of current, given real music (not test tones), even at reasonably loud volume levels, even considering peaks that can reach 100a-150a or more.

An alternator upgrade bears many penalties also... energy isn't free, a bigger alternator is a horsepower-robbing device.  It may not make much more idle-RPM current, it's current ability may happen at higher RPM's, making that horsepower penalty even less attractive. 

I'm definitely NOT one to jump to the "Oh, you should install a bigger alternator!"

Heh...  All Caps and Batteries provide is capacitance.

yes, that's true, but the point he was making (and you did a pretty good job of pointing it out there geoleman) was where the problem (if you can call it one) really lies.  basically, if your stereo isn't pulling the voltage down below 13.6 when your engine is running and you've got your lights and everything running, then putting a bigger alternator in is not going to make any difference.  i know there'll be someone who will argue that it will, but the fact is, it wont.  reason?  an alternator is NOT a constant (not even close) source of charging power - it is a pulsed source, and it's between those pulses that your system is hacking away at the surface charge built on your battery.  a battery is a good source of high current, BUT - the moment you pull more than about 15 amps from a battery, you lose about 1 volt of potential across the battery.   stop pulling the current, it goes back up - this is just a simple fact of physics when it comes to lead acid batteries.  you want constant voltage on a battery, get a lithium battery.... but anyhow - since you can't get a lithium that big, if you have a huge capacitor -- something in the area of 1 or more farads, you now have something that is able to stabilize (or buffer) the voltage between the gaps in the charge pulses from the alternator.  the other thing that the cap buffers out, is the latency that occurs between a sudden drain being put on the system, and the voltage regulator compensating by increasing the voltage across the alternators field coils.  now, if you put in a large cap, have a good battery, turn your stereo up, and it still drains, then your voltage was draining below 13.6 anyway (like i said at the beginning) and obviously your alternator couldn't handle the high beams, electronic ignition, fuel injection fuel pump, other engine electronics, those nifty looking lights in your wheel wells, charging your cell phone.  still - putting an alternator in that's 150+ amps i guarantee your lights will still flicker with a system that is just 300 watts - with the cap, it will eliminate the surging or minimize it as far as your'e going to get - short of switching your whole electrical system over to a digital supply for your stereo.  yes (sigh) i've done that in 3 cases where it was an EXTREEM system and the guy wanted every single last possible advantage he could have.  i wouldn't reccomend it for the average, or for that matter, for anyone other than someone whos got money/time to burn. i've seen the caps eliminate all surging AND it does increase the crispness and overall power of the bass response, not to mention, reducing distortion on your highs during those surges due to power loss.    so to make a long story just slightly longer, yes - if you want to improve overall quality of the sound system, plus get rid of the annoying surges (which btw burn brushes out faster on your alternator because the voltage regulator swinging up and down all the time), get the cap - it is well justified.