If I want to make my car louder, can I just change the exhaust pipe or do I need to change the muffler?

The cheapest route is to remove the resonator with a straight pipe. Most exhaust shops will do this for less than $200. If that's not loud enough you can either remove the mufflers or get performance mufflers. I'd go in stages here as you can always make it louder. Also look for a specific muffler or exhaust kit for your model. Alex