The big three is the first thing I recommend folks do as an electrical system upgrade. The factory charging and 12v distribution system is not designed to pass continuous high amperage. It's designed for short bursts of high amperage, such as starting the car. A mild car stereo system can draw more on a constant basis than the starter does. Voltage drop becomes a big deal then, when most amps are unregulated and become less efficient at lower voltages, resultant from the high amperage draw. Big three, then an alternator capable of easily producing the current to maintain sufficient voltage. Batteries should be after those two, because they don't correct the problem, they don't produce more...they simply store energy at the cost of "over clocking or over working" the alternator and simultaneously reduce the voltage the amp sees as compared to one battery.
