and Kappa Perfects qualify, you're facing serious decreasing marginal returns in such a noisy and sonically-messy environment as a car. You'd be better off ripping into the interior to dampen vibrations and remove rattles and such, and even adding noise insulation.
That all goes out the window if you aren't currently amping your speakers. If you've been running them off your receiver you might as well have whatever $20 pair of speakers is on sale at Best Buy...simply going to a halfway decent amp will be a huge improvement.