Oh boy are you opening a can of worms here. ....Yes there are better speakers and radios to be had without question. But your $100 gift card isn't going to do it . The sky is the limit here when going aftermarket stereo. You can spend from $250 to $25000 on equipment. It is all up to you how much you want to spend. First go to your local Best Buy and listen to speakers and play with the headunits and see what you like. In other words do some shopping. Find a local car stereo store and go and listen to some of their equipment and ask questions. Best Buy is not the place to ask questions though.
Like I said the sky is the limit. It all depends on your budget and what you like .
