I used to have all stock speakers in my 94 4 door accord lx until i replaced the fronts with infinity ref605cs components. All i can say is holy ****e...it was a HUGE improvement over stock speakers and im still just running about 16 watts RMS off the head unit and ONE rear stock speaker (blew the other). I play my music pretty freakin loud and no distortion at all from the front components (powered by a 4 year old alpine unit 16 watts RMS). The key to not "underpowering" speakers is not to automatically get an amp when you upgrade, but look at the power rating for what you want. My infinity's say 2-90 watts rms. Thats a good range for using either headunit or amp. Now if you shell out the cast for expensive stuff like top line MB quart, their stuff says something like 50-150 rms which means you WILL need an amp. Unless you plan on making a competition system i suggest getting some infinity components or ones with a wide range for RMS power.

BTW im getting some infinity 6x9 three way coaxials and once i get those installed ill let you know how "useful" back speakers really are.