A capacitor will solve your problem with your headlights dimming. Basically its the same as the battery but is designed to release charge much faster, and is perfect for huge drains of power like when bass drains power.
Make sure that your earth connections are big enough also. Often if the earth for the amp isnt large enough and isnt grounded properly you with cause an unnecessary drain on your battery. Unless you're pulling in excess of 800w RMS then your headlights shouldnt dim unless your current battery is shit, or your wiring isnt up to scratch. Replace your current battery with a deep cycle unit - Odyssey or the like. Use 8 gauge earth wiring on your amps for anything under 800wrms... 4 gauge for anything above that...
Installing a 2nd battery should be your last resort, and you would want to upgrade your alternator to go with it or you're wasting your time really. For cheaper you could make your current system run properly.. Your car uses bugger all driving along anyway, so if you're worried about being able to sit there for a while and play PS3, you're not going to get much benefit from having a SECOND battery... beause the first battery will be almost fully charged when you stop anyway..