I second what +fireamx said. Forget the exhaust if you're looking for performance. There are much better places to spend that money for performance. If you just want the exhaust note, try a resonator delete. You don't have to buy a whole new cat-back exhaust to change the sound.

Changing the stereo is an excellent bang-for-your-buck mod. Before you change the stereo though, try adding a set of bass blockers (capacitors) to your door speakers. For less than $20, your stock stereo will sound a lot better. If you're looking for features, then replacing the head unit is the way to go.

Getting your ECU tuned is a nice mod. Without any other performance mods (such as the cold air intake) the tune isn't your best bang-for-your-buck. Yes, your car will run better and have some added pep. But a tune really shines when combined with other performance mods.

The TVT performance packages are a good deal, however it can be daunting to spend that much all at once. Check the group buys sub-forum and get the best price you can.