Ive got low end polks in my car the 6.5s and they sound pretty good. If i could by another speaker i would save up and get a component set for sure. Mids are just soo much clearer with them, but mine arent amped yet so....
i would hold off until you can afford an amp. you won't get that much improvement powering those $150 speakers with the head unit. An $80 set of components powered by a little 2 channel amp with 50 watts RMS per channel will sound infinitely better than any speakers powered by the head unit.