350w rms is the max you should go for a 8 gauge, if you are runnin a 500w rms amp, go 4 gauge, and there is room for expandibility... personally I ran an 8 gauge... then got another amp and sub and bought ANOTHER amp kit so i have two 8 gauge runnin from the battery looks fugly and if i get the desire to make my trunk look pretty, i'll go and buy 16ft of 4 gauge and a dist block.

sound quality is based on the insulation of your RCA wires and what gauge wire it is. if your stereo needs lotsa power and you're running 10 gauge or something, it can't get all the power it needs, which equals bad sound and/or somethin is strugglin and could damange something. also, when you run the RCAs run it on the other side of the car opposite power lines, less interference when power is runnin through it. the quality of RCA cables also mean a big diff. in quality of sound

SO... ... CLIFF NOTES:

-get good quality RCA cable (you'll probably need 16ft if you mount amp in trunk)
-get 4 gauge amp kit
-make sure you have the right fuse
-mount amp right side up, don't put it upside down
-don't blow your sub cuz your amp supplies more power (unless that is RMS rating)