Honestly, as far as a big increase is concerned, I would buy the supercharger kit that GM Performance Parts makes for the Grand Am's and put a smaller pulley on it. You can find the kit on their website. Of course if you want to go a cheaper route you can find the parts off any GTP model(junkyard, craig's list, etc). If this is too expensive for you the first things you should do to a car is get a better cat-back exhaust and a cold air intake(not just a K&N it will do nothing).