If you are going to get a sound system buy it used, there's a ton of stuff out there on Craigslist. Also attempt to buy halfway decent stuff and don't go for the headunit with a bunch of buttons, lights and motions...it'll just break. You must also remember too, if you have a stereo and people know about it, you'll probably get it nicked.
I had two sound systems in my Blazer, one was stolen and the other went with the truck when I sold it. The second one was much better than the first one and I ran mostly Infinity stuff. It sounded great and the bass was very good without having that annoying hammer sound. If you are going to put a sub in your car, I'd recommend the Infinity Basslink. For $300 you get a 200W amp and sub in one unit, it's also quite compact so you don't ruin your truck/hatch space.
I'd stay away from exhausts, they don't do anything unless you have other parts to go along with it. All you'll end up doing is having a car that sounds like it has a hole in the muffler and you won't be any faster.