what subwoofer power

I have a Boss car stereo that runs two 8" 40 watt speakers. But I want to add 'one' sub-woofer in simple box. what do I look for?? A dual coil sub-woofer? And what kind of wattage ?? are the watts listed on the speakers the max it will take or should I look for an 80watt woofer?? I think this...