2800w + 2000w =4800w.
x .85% efficiency (which is a rough guess) = 5,647w
5647w / 14.4v = 392 amps.
Thats when it's hitting hard and playing at max. The rest of the time, between it is charging at a rate higher than it is using. If you don't play it on full, and don't listen to medium heavy bass, and don't listen to it for more than an hour at a time, with a little shut off time before you turn the vehicle off to make sure you recharged, you could probably get away with 250. I would definitely go 320 on that so I wouldn't be limited.
That power is just your stereo. You also have the rest of your vehicle. Ecu, fans, and AC are big energy users, if you use those often. Injectors, lights, turns, anything that connects to your dash uses something. If you've added any lights, if you want to add another amp, or change one of them. If you want to charge a couple phones, an inverter, a screen, etc.. Just seems an easy answer to buy the 320, and not be limited right away. I would definitely go 320 with that. I'm at 240 and half that, because I wanted some room.
Personally, electrical is the one thing, if you are going to upgrade it, IT'S DEFINITELY WORTH THE MINIMAL PRICE INCREASE, to upgrade to what you MIGHT need, and not just what you need now.