alternator

A cap will not add any power to your system.  All it does is smooth out the voltage ripples (peaks and valleys).  You wouldn't notice as much a voltage drop when the bass hits because the cap would discharge/recharge itself as needed to maintain a more constant voltage.

If you do actually plan on adding a 2000+ watt RMS system, then I'd get the alternator with the most output at idle that I could find.  Yes, as boulderguy started to say, you should upgrade the lead that runs from the battery to the alternator, as well as the factory grounds (chassis to negative/body to negative).