Okay, so I've been using a borrowed amp for quite a while and I was thinking about buying one of my very own, but I don't know exactly what to look for. I know that I would probably like to get a 5 or 6 channel amp so as to be able to hook up a subwoofer, and it would be nice for it to have a builtin crossover so that the subwoofer would only get the low frequencies, but what I don't know is what sort of wattage I should be looking for. All of the things that would seem to be common sense seem not to apply. My assumption that higher wattage would mean greater volume, all other things being the same, was shot down a while back. I don't really have any other guesses, so someone tell me what I should be looking for.

(As another point, how should subwoofers be mounted? Should its enclosure just be put in the trunk, or should there be some port from the trunk to the cabin?)