If you decide to buy adapters, buy them online at places like ebay. I almost had to pay 30 bucks at Circuit City to buy the car harness and the stereo harness. It's nice to have the adapter if you don't want to have the trouble when switching your stereo with a new one, but if you're a good DIY, then there's no trouble, but it's a bit time consuming. Make sure you disconnect the battery though!
http://www.installdr.com should have the wiring info you need.