pretty much what everyone is saying:
-in a perfect world, you want your car to always be charging above 14.0v while system is at full tilt.
-adding batteries can protect your voltage for a short period of time, but there is a time where you can play it to long and the alternator can not keep up with charging the extra batteries.
-some are happy with being in the 12 volt range, but that is completely subjective, the higher the voltage, the less hard your system needs to work, which results in less heat, which also results in a more healthy system.
-if you have the money, go with a HO alt. that is the only way you can completely cure your voltage problems.