get something with a high aH rating and cca. how much do you have to spend? are you only trying to use one battery under the hood or are you willing to put one in the back with the system too?
I would suggest a deka intimidator 34/78 battery. pretty inexpensive and well worth the money.
wiring a second battery is easy. just run a length of wire from the postives and negatives of the front battery to the positives and negatives of the front battery. then wire from the second battery to your amps and done.