Improve sound quality
I would say that just the headunit will not make much difference. It's almost always the speakers that deliver the most bang for the buck in car stereo systems. If you choose high power speakers, you may have to get a new headunit/amplifier that can deliver enough power to drive them properly.
As an example, I replaced the 4 DELCO speakers in my last car (Olds) with a mid-price set of Pioneers, and kept the stock DELCO headunit. What a huge difference in sound quality. I ended up replacing the headunit as well, but it did not make much difference except that the new unit had many more features (CD/Tape/Radio) and better bass handling capability because it was rated for 400watts.