Intake header and exhaust is the basics. Cams are great.. but can run any where from $300 to $800 for a good set. I'd say the single best thing with a small amount of money is getting your car tuned. A good street and dyno tune is going to bring the most out of your engine. My friend tuned my other friend's LS and picked up 22whp and he said the car felt totally different, and it pulled all throughout the RPM range.

Tell us what your budget is and we can best help you.