If you are planning on doing any modifications that require tuning, you would want to go with any year other than 2008. For 2008, Pontiac switched the Grand Prix from serial bus to can bus communications, and due to it only being a one year change, tuners have not taken the time to do the development to make it tunable.

In 2006 the GTP sub-model was dropped. 2006-2008 had a standard (non-GT) Grand Prix, a base model and a GT. The GT generally came with more features than the non-GT and base models.