upgrade alt and get a 2nd battery.

if you're TRULY grabbing those wattages -- If you're planning on maxing your system out all the time for competition (which i really doubt) you'd probably need all three, your alt is definitely going to see too much demand and will not sustain those power requirements.

2nd battery will help prevent killing your main battery.

But this all depends on what you're using it for.

The capacitor will only help you for those quick bass bumps (non contiguous) and the 2nd batt will just be a pro-longer, better used for when you listen to your system while off.

Basically the only thing you REALLY need to do is upgrade your alternator and upgrade the big three. You should be fine after that. Make sure to do a lot of testing beforehand to see how your car reacts to the changes so you don't get yourself stranded somewhere or the car dies in the middle of a loud, bass intensive song in the middle of an intersection.

Toby and Dalton will pipe in here relatively soon to tell me that i'm only giving a general and not very accurate answer and will proceed to type up an essay on the subject.