I have always heard 1.0 farad per 1000 watts. I would spend the extra to go with the 1.0 farad cap inorder to save myself the trouble later when and if I were to upgrade to a larger amp. Also, a larger Cap cannot hurt. It can actually help by making more "clean" power avalible durring the big base hits. This may equal better sound as well as less strain on the power and charging systems of your vehicle.