4 things to consider:
1) will adding more power blow the stock speakers.
2) if you're upgrading your speakers, or it won't (doubtful) you will need an amp with high level, or speaker level inputs. THis requires you, usually, to bypass the stock amp by cutting the wires and splice the wiries for the high level inputs...which voids your warrenty rather nicely.
3) If you want increased sound quality, without removing the head unit, the best thing to do is to change stock speakers ith your amp. Amp's main purpose in life is to amplify the signal, ie, give power, allowing you to play your music louder and clearer without discernable distortion, but that extra power is no good if you don't have the equipment to handle it correctly (the aftermarket speakers).
4) For best results, seriously, change everytihing, head unit included. Find one that matches your car interior.