It all depends on how big you want to go. I'm running my factory alternator and my system has never had any problems, my lights don't dim at all even at full volume, and my voltage always stays above 14V. My one Audison amp provides over 1500 Watts of power with a maximum current draw of 110A according to specs http://www.audison.eu/index.php?page=product&id=13
I have actually metered it and found it'* putting out about 5% more wattage per channel than what it is spec'd at. The second battery is really more important in my opinion. My main battery died of old age this year (It was 8 years old) so I replaced it with another solid state battery from stinger. Another benefit to these is no more need for the hose to empty gasses from the battery outside of the car.
I'm not saying the alternator wouldn't help because it surely would, but I never had to touch mine and a battery is a lot cheaper than an alternator...