Not too bad at all. The 04-08's are getting tuned and modded more nowadays as they're getting cheaper. The tuning software out there (DHP, HP Tuners) will work on any GP up through '05.
Here's a good start: http://www.grandprixforums.net/threa...ding-Your-3800!
As a general rule, do maintenance first, then supporting mods, only then do you drop pulley sizes.