Now i know the benefits for reliability and for car stereo improvement, so I planned to replace everything with 4 guage (bat-alt, bat-ground, etc). but my friend, into major SPL, says i should go for 2 guage or thicker. Is there such a thing as having too thick a wire for this application?
You can never have to thick a wire. It gets much harder to work with when it comes to stripping, and cleanly running wires to where they need to go. I don't know if you would need to upgrade the battery wires. Just when you run wires to your amp make sure it is a good 4 gauge or better. The stock battery wires should be fine. You may want to do a grounding kit instead of replacing all the stock wires. I have been playing with Hi Powered Stereos in my cars since the beginning and have never needed to do anything with any of the Stock Battery Wires.
well, i got one of those Scoche thingies for the caps to make adding in wiring later easy, is has 2 4-guage and 2-6guage holes. But I don't have a cool stereo yet, i was mostly worried about engine performance being affected.