Higher Wattage doesn't always mean better sound. It only refers to the power output (volume)
If your headunit outputs 45w RMS per channel and you have 50w RMS speakers in there already, just putting in a 100w RMS pair of cheap speakers is not really going to improve your sound...
I would suggest buying some decent Alpine or Infinity components, with the hope to one day put an amp in the car to power them!