I recently had to change the tensionor pulley (located under the alternator) on my 2004 Grand Prix GT. The first thing to know is that the bolt on the pulley is reverse threaded--righty loosy. A 15mm socket will do the trick. I needed to put a bar on the end of the socket wrench to give enough leverage to loosen the bolt.
The serpentine belt looked in good shape but I will probably replace soon since it is cheap and easy to do. Replacing the pulley was only about a 20 minute job which includes taking off the coolant overflow tank. The Grand Prix engine space is a bit cramped and you get the opportunity to easily check your power steering fluid while you are doing this job.
Auto Zone and Advance Auto parts store. Rock Auto discount supplies. Pep Boys. Tires