You also need to remember that any modifications which increase power will also put more stress on the engine and transmission. If you do an effective job of hopping up a 5 year old car with an automatic transmission, and then use that power frequently; the transmission is likely to fail sooner than it normally would. The engine may also go bad more quickly; depending on how much you increase the power, and what kind of care and maintenance the car has had. This game can become expensive very rapidly. If you have a limited budget, need the car for daily transportation, and cannot afford to risk unpleasant surprises; I recommend not getting into this. Expensive things are bound to break.
As far as doing an effective job of increasing power; you need to know that any of the common means of hopping the motor up (bigger cams, compression increases, head work, higher flow intake and exhaust systems, spark advance and fuel delivery changes) will also change the emission levels enough that it will trip the check engine light and fail smog tests. And that will lead to the car becoming illegal to drive on the street (and also illlegal to re-sell). Can you afford to have this happen to your car???
Texases was right; the cheap modifications like exhaust or intake will make noise; but will do little or nothing for performance.
There is only one type of modification that will produce substantial power increases, will not make the car illegal, and does not cost a fortune. It is called nitrous oxide injection. Nitrous kits are available in many different power levels. The lower power kits do not require major retuning; but the more powerful kits must also have an ignition retard box and/or water injection added to prevent the engine from self destructing. The downside of nitrous is that it can only be used for a short time before the nitrous storage bottle needs to be refilled. That is inconvenient, and also costs bucks. And the high power kits will rapidly wear out an engine. But it is the easiest way to make a stock looking and stock sounding car be able to beat people.
www.summitracing.com has all kinds of nitrous kits and equipment, at very fair prices. If you decide to go this route; I strongly recommend you first buy a few good books on nitrous injection; and read them carefully. And be sure to retune the fuel and ignition systems to match the amount of nitrous that is added.