Since you are going to college...

Keep in mind that wheeling can get expensive (especially if you start breaking stuff or getting flat tires).
If you plan on doing somewhat frequent wheeling, I'd get skids and sliders to help protect your X and prevent trail damage (and more $$$ lost paying to fix said damage)

Since I'm guessing you'll be on a tight budget the next 4 years, I'd do the sound system.
1st, it's not going to break and need repair (unless some drunk person vomits on your system)
2nd, you'll find the stereo a much better expense if you aren't wheeling your X. chicks dig trucks with sweet sound systems.
3rd - I second the motion by armandov about the exhuast and headers

A lifted X looks cool, but does eats up gas a little more and (depending on where you are going to school) could make garaging/parking the X a little more "fun." For Example, if you are going to college in the city of Boston- With just a 3" Sus lift (on my '01 X), there are no public parking garages in Boston my X will fit in (unless I take the stock roof rack off)...
so keep that in mind

Sliders and skids are a good investment, but if you aren't going to be needing the use of those items over the next 4 years (because of a lack of time to wheel)...then I'd spend my money on something you will be using

just my $.02