I would have to disagree with you guys and say do the speakers first. I Left my stock head unit in, and put 2 JL Evolution 6X9's in the back, and 2 Alpine 5&1/4's in the doors. The sound quality difference was huge! The bass coming out of the JL's even with the stock head unit was great. Of course, if you REALLY crank it up, you will still get a bit of distortion, but I noticed a huge diff. when I changed the speakers. But of course, when you finally change the head unit, the sound will be better and louder. I know everyone else says do the head unit first, but if you have a million dollar stereo and crappy speakers, you are going to get crappy sound! The speakers make a huge difference in sound, and although a high quality deck will make a difference as well, the difference between a stock head unit and an Alpine head unit for example, are not as large as the difference between stock speakers and some good JL's or alpines!
Try the speakers first. Trust me, you will be happy with it.......but then 2 months later you will want to spend 2 grand on an entire new system like i did! ha! so much for university money...