do i need a power capacitor, what is farad, what benefits will one provide? a little help please.?

just installed a new audio system in my car,
I'm using a 4 x 150 watt amp with a 2000w max output. sounds great at the moment but my friend recommended that i get a power capacitor.
i've had a look around but i don't relay know what I'm looking for. they seem to range from 1 - 50 farads.
what is a farad?
do i even need a capacitor?
what benefits will it provide?
are there any modifications i need to make to my car? its a 2003 mini.
i would greatly appreciate your help

thank you

Doug

you do not need a capaitor.if your system is running fine with out voltage drop then u are ok do your lights dim bad when system is running if not your ok.capacitors i dont beleave in them..if you do the big 3 you will have more power flow and better current running to your amps the big 3 really makes a big difference..id go that route.or get a 200 amp alt with the big 3 and youll pound all day long with no problems..capacitor is useless in my opinion..i think youd do just fine buy doing the big 3..steve