[sr20de4evr]

Upgrading the big 3 is quite possibly the cheapest, easiest, and most effective thing you can do to help out your electrical system.
And 2 8ga is definately better than 1, but really you should have atleast a single 4ga, if not 2 4ga or a 1/0ga.

With electricity you basically have 3 things you need to be concerned with, current, voltage, and resistance. Resistance is measured in ohms, and it really doesn't apply to the electrical system of the car, so I'll just skip it. Your alternator is the thing that powers every electrical component in your car. It turns the rotational motion of the engine into electrical energy and sends it to your battery and everywhere else. It will generally put out between 12.5-14.4V, and as much current as required up to a point. Most factory alternators put out a max of around 80 amps (atleast in sedans). This means when your alternator is working it's hardest it can only supply 80 amps to the electrical system, if whatever is sucking all this power (your amplifiers in this case) require more, then they start pulling the extra current they need out of the battery. If your amplifiers suck too much current for the alternator to handle then you could run your battery dead pretty easily if you aren't careful.

An 80 amp alternator, which is probably what you have, should be fine up to atleast 1000-1200 watts though, as long as you don't go blasting bass music at full volume everywhere you go. If you have dimming problems then upgrading the big 3 will most likely help a great deal, if not fix the problem entirely. If it doesn't fix it completely then your next step is normally to get a new battery (unless the one you have is new and in good working order). After that a new alternator (one that can supply the current your amplifiers need) or a cap (sort of effective for short bursts of power, basically like adding a quick discharge battery, pretty useless though if you have a tendency to run the battery dry) will come.