e36 amplifier upgrade

Can anyone provide some guidance as to how I would replace the stock amplifier in my e36 with a new amp? Should I be looking for a four channel amp and wiring the rear tweets and mids to a single rear output? Similarly, what about the fronts where the "woofers", the mids, and the tweets are all wired separately? I guess I'm trying to decide if I need an amp with more channels...