I ran 4g from the battery to a distribution block under my drivers seat. From there it spits into 8g to each amp. I have a 1000w rms mono amp under one seat and a 350watt rms under the other seat. Make sure you ground both amps to the same spot to help cancel any wierd noise.
I ran 16g speaker wire to the front speakers, there is a thread on how to pull speaker wire into the front doors. 16g is fine for the speaker wire unless you are going to be running some ridiculous amp, alot of guys go way overboard when it comes to wiring.
I have no problems with power or sound in my car.
If you need any more info, let me know and I can try and help you out.
Matt