Couldn't agree more. when i bought my car, it had Polk MOMO comps in it running of the HU. When i bought my Memphis 16-st-3004, the difference was indescribably awesome. Priorities- 1-Amp, 2-SpeakersDo your speakers run off your stereo's amplifiers? If yes, that is your weakest link right now. Install an amplifier first, then change the speakers. Even cheap $50 speakers can have a night and day difference when running with more power. For a simple setup, I would recommend a decent 4-channel amp. Bridge the rear channels for a subwoofer, use front channels for the front speakers.
Regarding speakers, if you can spend $250, I would recommend to try HAT Imagine speakers. They are convertible between component and coaxial. A lot of people here like them.