Radar detector ! You will find yourself driving your car faster and harder when you have some protection against the cops.
All kidding aside, I would definitely invest in a really nice cold air intake, such as one from AEM or Dinan. It won't cost the full $500 (so you'll have some loot leftover for that radar detector
), and you can install it yourself so you don't have to spend any more money on installation. It will also give the car a more aggressive tone under hard acceleration.