Basically here's how it goes. In order to run a subwoofer you will need an amp. Some subwoofers (Infinity Basslink and Bazooka tubes) have built in amps, but for the most part you need the actual woofer and the amp. Its not all about how loud it is, or how easily it'll rattle your car about. Its more about how clean it sounds, and how powerful the bass is. True bass response is quick and powerful, which is rarely found in the cheap setups. You get what you pay for. I am a strong strong support of my Infinity Perfect subs. They not only hit very clean, but they have enough power to make it sound good. Also its all about how you have it setup. You obviously need to taylor a sound system to the kind of music you listen to.
If money is not an issue I would get JL Audio 13.5W7s

But with an amp thats a quick grand.
I truely believe that the (2) Infinity Perfect subs and (1) JBL BP600.1 setup is one of the most cost effective and efficient combinations. The subs are around $140 shipped each and $195 for the amp. You can't beat $500 for a top of the line system.
Missay, you may run into a little more complicated setup if you plan to keep your stock cd player, but its still doable