Thats a lot of watts, and i would recommend getting a capacitor i suppose. I wouldn't worry about the battery unless your going to play your system with the car not running. Capacitors tell you how many watts they are rated up to, and personally I’ve never seen any that were rated past 1,000watts, but i suppose if you can't find a 1,100watt/1,500 watt capacitor then a 1k may due the job well.
Personally, i would install the stereo and have it tuned to see how everything works out. You should try to reduce voltage drop first by sellecting the right gauge power wire and keeping the power wire short as possible (i suppose you've already done so). If you get constant lighting and electrical problems then a capacitor won't help, but if your lights dim on big BOOMS then i would get a capacitor.
I've had a capacitor in my first car before and it helped alot. My amp was 500watts powering the "holly-favorite" of mine OZ subwoofer--no more flickering.