the biggest thing you could do before adding a second battery is add additional grounds -
from battery to frame, from engine to frame.
we all add nice thick power wires for our amps on the positive side- but electricity travels in a circle (in bascic terms) and forget about the ground -
I have fixed many problems with customers cars that outher shops could not figure out just by adding a 4 gauge ground wires. (can eleminate engine noise, dimming headlights) and make all electrical devices in car work easier.