headlights dimming Q

i have a 91 mr2 and if my car sits for a day, it'll hardly start or if i leave the radio on for like 10 mintues with the car off. Could i just need a new batter or is it my alternater to.. I did have a systsem in it and whenever the bass hit, the lights would dim alot. I've also noticed sometimes at a stop, when i put on the brakes, the rpm's drop like 200. If an i need a new alternater, what amp is stock and what would b a good size to go with. and for a new battery, i'd like to get a optima yellow top. Thanks, u're helps greatly appreciated.

It sounds like your battery, try replacing your battery with another batt. out of a different car in your family. See if that fixes the problem. If not, its your alternator.