You should be buying a higher output alternator before either a cap or a battery if you need the extra juice. A cap is going to smooth out your high juice spikes, but you're still actually loading up your alternator just as hard. The alternator is what makes the power that your car (and system) uses, not the battery, and not the cap. The battery and cap STORES the power for when you need it. Deep cycle batteries help cause they give a real flat output and can take a whole lot of abuse.
now just buy a battery. ITll do better than a cap. BUt if youre going to be entering contests then you would need to buy caps because you have to burp the car i n some comps with the car off so you get no help from your alternatro. So you would need batteries and caps. BUt if youre a daily driver, then i say just buy a deep cycle battery liek a optima yellow top since you already have a HO alternator
I'd do the cap personally. The Battery will help mainly when the car is off, a cap helps control transient peaks and dips, something a battery cannot do. When the car is running and a voltage bias is applied all the battery does is charge anyway.
I was actaully thinking about getting a cap and a redtop for under the hood. How may farad cap do i need? I have a eD NINe.1 and a ed NINe.2 amp.