Adding an aftermarket amp is definitely an improvement.
What you want to do is get the amp you want (I have a JL XD/400-4), then match up some speakers to it. For instance, you don't want to have a 4 ohm rated amp and install 2 ohm rated speakers. Also, it's good to have a bit more wattage (RMS) in your amp than the speakers require.
I have a set of Hertz HSK 165 2 way component speakers in the front and a set of Kicker 6x9's in the rear door. To top it off, I have an Alpine MRP-M500 single channel subwoofer amp pushing an Alpine 10" Type S subwoofer in a sealed box.
I like how it sounds.