If all you want is a 5-10 hp gain, you can try something like a CAI, exhaust header, or perhaps even super-lightweight rims. But if you want a lot more power, like that produced by a good turbocharger, or reworked heads, intake, performance camshaft, etc., and you are prepared to spend the thousands of dollars for those serious upgrades, sell your car now. Take the proceeds and the thousands of dollars you would have spent in upgrades and buy yourself the performance car you want. It will be much cheaper in the long run.