That will depend on your car. Some may come with 160A, but it doesn't mean your stereo will get all of it. In fact, you'd be lucky to get 80A of free power for accessories. Install the system first, and if you see signs of car struggling, then get HO alternator.
I think it would be a RARE car to require 80A of current...maybe with EVERY accessory in use -- lights on, pwr seats running, windows rolling up and down, etc.
Even if that WERE the case -- 80A of untapped current is a BOAT load of music power. We're not talking about playing sine waves here.
nathan from excessive amperage wants to sell me a 160a HO alt. and im wandering if i should buy it or not. i would buy it if i knew i could run 1800wrms+ off of it.
i'd say if you told him all the power you'll be running and all your vehicle info, and he recommends a 160a alt., then i'd take his word for it. don't think he'd try to screw you. but why not step it up to about 200a, that way if you want to upgrade, you have the power for it already.