Theres a few things, such as : K&N Filter, Uprgraded Ignition ( and not just plugs and wires, but a new/ higher output coil, igniton box, such as an MSD 6A or 6AL, ect...), Cat back exhaust will run you about $250, which is about what i paid for my 3 inch Flowmaster, headers are also pretty cheap, seein as how you dont need AIR tubes, you could pick up some Holly FlowTecsh for about $90. Out of all these i would do the filter ( if you havent already) the headers and the exhaust system, the exhaust and headers will really help you notice your cam, and improve the way it sounds as well!
Edit: whoops, kinda spaced on the part where it said you already had headers, my bad. in that case, if you havent upgraded the rest of the exhaust system, then goin with larger diameter pipe and a good muffler will help uncork some of the power lost after the headers.