A single (well powered) sub in a properly built box, paired with a nice set of mid-range speakers/components up front should give good sound without compromising weight too much.
Keep in mind there are extremes to both sides.
If weight is an issue, take out everything that doesn't help the car go, stop or turn or keep you in it safely.
If weight is not an issue, install as eloborate of a system as you can afford and install sound deadening/dampening materials.
So forget all but my first sentence and have fun
