Try going to http://rpmoutlet.com
as they have some great combinations of add-ons which they have tested, and as another poster said, the air intake, and exhaust are the cheapest upgrades. You can add about 50-60 HP with an air intake, a larger throttle body and elbow and even more with some headers. You can spend about $800 total on the Intake and a few hundred on the headers and take a 300 HP engine to somewhere between 360-400 HP. Of course , if you want to spent a few thousand for supercharger, you can end up in the 400-500 HP range.