Don't forget two powerful 4 channel amplifiers. Power the 6 speakers with 6 of the channels and then bridge the last 2 channels and that to a sub. It would sound real nice if you hook it up right.
I just have my factory speakers plus one crappy a$$ sub (12" super pro red surround), I had two till i blew the other one out...

Anyway, my stereo sounds great! So before you go butchering your car think about all the alternatives, and/or equipment you're going to have to buy.