so my first car was a jetta also. Although mine is a 2002 Jetta GLS TDI (diesel). I replaced the head unit with this pioneer one...
http://www.crutchfield.com/S-q58nUW7...H-P6000UB.html and the stereo has MUCH more bass and low end now. The one above is discontinued for a newer model most likely but a comparable one will give you similar results. Overall is sounds WAY better than before. So if you want a better stereo, just get a good headunit. The extra power it puts out will make it sound much better.
The only thing i had to buy besides the head unit was a new harness.