Hmm your question depends on what you want from your system. Do you want to compete?? If so then big amps and many big drivers are needed. Do you want good sound that gets loud enough to drive all but the insane out of the ca?? In this case 150-300 watt amp feeding a single or maybe 2 10" woofers should do the trick. The factory HU sin't all that hot as a source period. If you really want good sound upgrade to an aftermarket HU with 40 watts per channel or so (or even better an HU and a second amp at 40-75 watts per channel), and upgrade the factory cheapy speakers with something better. All depends on what you want and your budget. For alimited budget start with the sub and sub amp and add other stuff as you can. A good order would be (sub and sub amp), (Headunit), (speakers), (4 channel amp for speakers).