First of all, caps are useless they will not help your electrical system out at all, you have to upgrade your alternator. Upgrading a battery will be pointless because it only is in use when the car's ignition is off, in my opinion, get 2 12's and feed them 750 watts each and that will be more than enought to blow your brains. Then get 1 good 100x4 amp to power some great components and you will be set. DOnt forget to upgrade your alt!
2k4 330ci on the way