If I'm running an 800watt amp is it a good idea to upgrade the alternator?

Probably not required.

Factors:

Is this a lower quality 800w peak/max amp or a good quality 800w+ rms amp?

Size of the existing alternator.

Expected listening characteristics -- blasting it all the time or listening at low/moderate volume most of the time.

I've been running 1000-1500w (rms) systems for close to 10 yrs now in cars with 75-130A alts. I have never had any ongoing electrical system problems. I don't blast it all that often.

I DO generally burn through an alt every couple of years, but I don't mind turning a wrench either.

FWIW - Brad is misrepresenting the function of a stiffening cap in a car electrical system.

Besides - it's not the up/down that's hard on the alt, it's too much UP. High current requirements means a lot of work from the alt. A lot of work from the alt means a lot of heat. A lot of heat a lot of the time means a high probability of a damaged alt. A cap can't change any of that.

Upgrading the alternator is really never an issue that you must tackle on mid grade systems. All you really need is a capacitor. Alternators typically are used at around 70% of their rated capacity

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Modern alternators usually put out between 95 and 130 amps. That is plenty. Audio amps take juice in pulses, so it puts a strain on voltage at the battery when the bass "hits". That drop in battery voltage creates the dimming lights. The regulator increases the alternators output, then bass stops for a moment, so the voltage regulator decreases alternator output. That increasing and decreasing charging power is what is hard on the alternator. That happens with every "bass" hit.

The capacitor stores the energy like a battery, but is able to discharge and charge itself much faster. This allows the capacitor to store a large amount of energy, output the required amount of electricity at any given moment, and then allows the alternator to slowly charge the capacity. All of this is effective at getting rid of the dimming lights.

Long story short.. unless your alternator is failed to the point where your battery dies while driving down the road, it should be fine with an added capacitor.

If you have an 800 watt amp that does not mean that that much is going to actually be running through your speakers. Typically about half of that to a little more will be produced. I currently am running a 2000 watt amplifier with my 1200 watt speakers. I also run it with a capacitor. It acts as an alternate energy source so that it is not recieving all of the power from the battery that way everything electric in your care performs better. All though a new alternator would be good it is not ideal. Go to your local autozone or car repair place and they should check your battery to see how much energy is coming from the alternator. I believe that a good number is around 14 amps.

No, you are fine.