If i have an audio planet 3000 watt monoblock amp what capacitor is good do the farads matter?

A good minimum is 1 Farrad/1000watts. If your that serious then make sure your running 1/0 gauge wire for positive AND negative all the way back from the battery (and from the alternator to the battery -your factory 10 gauge wire isn't good enough). A cap is not a waste, it CAN (but of course nothing is ALWAYS) save your alternator by providing a buffer between the alternator and the amp. This buffer is important because an amp will suddenly increase demand for power (like when the bass hits) and an alternator can't supply it so you can fry the alternator with constant or regular use of a big system by repeatedly straining the alternator by trying to take more power from it than it can produce. Keep in mind though that a cap doesn't increase the power output of your system, it still all comes only from the alternator.