What farad cap if any should I get?

Without knowing more about your system its hard to give you the best answer. a bat cap is never useless. before you get into changing your alternater try a bat cap and replacing your battery with an Optima yellow or red top. If you are running over 1000 rms watts I would get 3 or more farad bat cap. Ebay has sweet deals on bat caps. If you are worried about damaging your car, those two should eliminate your problem. If you are running a monster system....say over 3000 watts rms I would change all 3. Your best investment is to buy a car audio magazine and see what the professionals are doing. Crutchfield.com is also a very good site for info on what to do