Battery struggling

I have recently invested in a larger capacoty battery (60 odd ampere hour), however when i have the ice cranked up at night the headlights dim slightly[}:)] so i have one of two options: - Upgrade the alternator on the car (what will this cost), add a split relay and use a second battery in the back? Or add a second one farad capacitor (as the one currently in use is only linked to one of the two amps), or should i link both amps to this single capcitor?

split charger will be easier with another battery
:D

<font color="black">It depends what you are running in your car, I have a directed amp, 2 sets of speakers and a 10" Alpine sub in the boot and Im still using the battery I got with the car (2nd hand, 1.5 years ago), even on high bass external lights are fine.....</font id="black">