Do I need an amp for my truck?

I'm buying some subwoofers for my truck, because it really has no bass. They're going to be two 12 inch 300W woofers. I know a lot of people get external amps. I already have a pretty nice stereo that wasn't factory. I know some about car stereos, but not much. Do I need to get an amp, or maybe should I get an amp? What would be your recommendations?