Unless you're planning on running the lights with the engine off for extended periods of time, an additional battery is going to do you a lot less good than just a larger alternator will.

If you do actually need an auxiliary battery, the way I would handle it would be to pick up a 12V deep cell with the most amp-hour capacity you can afford. I've had great luck with Odyssey Group 31s, but they are pricey and unbelievably heavy. You will need to isolate this battery from the rest of the electrical system at least when the car is shut down, so that using the aux battery's capacity doesn't drain down your starter battery. This can be as simple as a mechanical battery switch or something considerably more sophisticated and automatic. If you want to go a more hands-off route, have a look on Expedition Portal at some of the systems people are using on there to isolate their camper battery banks from their starting batteries. There are some very sophisticated and very clever solutions people have come up with.

If you are planning on running this aux battery hard, you still may want to consider a higher capacity alternator, since it's going to be working hard charging up both your aux battery and your starting battery, as well as providing for the considerable electrical load that the car wants whenever the car is running. Big electrical systems are hard work on small stock alternators.