a sub will give the sound more bass without having to distort the standard speakers but to make the system sound better, like has been said, start off with the head unit, you will have to change this to have the outputs for the sub anyway, then upgrade your door speakers with some better ones, the sound will be soo much better than the paper cone speakers you have at the moment, it will mean stripping the car out a bit but you have to get the edges of the carpet up anyway to put the sub cables in .

i give FLI the thumbs up for cheap audio, they are vibe re-stickered just at affordable prices, i had FLI in my car and it really is good value, i gave it beans everyday for work and back, and they all held up lovely.