In my opinion, buy a stereo for your car. It's something which you'll likely get much more use out of than an exhaust. In some cases adding a larger exhaust will actually hurt performance since (in essence) the exhaust system and the intake system of a modern car are balanced.
In order for a performance exhaust to make a real difference in HP you need to start at the intake and work your way back. Adding a less-restrictive exhaust without (at minimum) opening-up the intake with a less-restrictave air filter, then cylinder heads and you'll still have a bottleneck at the catalytic converter. In short, adding a high-performance, free-flowing exhaust without doing the whole package would be like wearing one super-expensive running shoe and one cheap one and expecting that wearing that one shoe will make you a better runner.
Plus you'll be able to turn-down a new stereo system if your neighbors decide that it's too loud. Hard to do that with an exhaust system :laughing:
Kevin