6x9s blow.... speakers should be round, not ovals. If I only had $500 and didn't want to give up bed space, I would first replace the stock front speakers with some 6" / 6.5" coaxials. (mine are JL VR600s for less then $100 shipped) Then I would switch the rears speakers to 6" speakers (adaptors available online or easy to make).

Hard part is finding a good spot for an after market amp, which you MUCH have for any sort of power. Do a search for "amp under passenger seat" and see what others unwilling to lose cargo space have done.

If you are willing to give up some space in the back, you can go with something as small as a quality 8w3 microsub in a ported box. I had one of them in my S10 behind the seat and it really had some power.