If you want the biggest bang for the buck, a performance Camshaft would be it. Just make sure your engine is in good condition with no knocking, ticking, oil burning, etc. Otherwise you will be throwing your money away on a new Cam and lifters. Also, don't just pick any large Cam. I have seen so many people do this and they ruin the overall performance of the engine. Call a reputable company like Comp Cams and have them match a Cam. They will ask you about the engine, the transmission, rear end, vehicle weight, etc. to get you that matched Cam.
Headers and exhaust would be the next thing to do.
