Whats next. ('08 GT mod advice needed)

CAI and a tuner not a chip. Predator and SCT Xcalibur II are the most popular, buy them based on who you plan on having doing your tuning I would say.

If you don't have to get the car inspected then throw on an offroad X or H pipe and a axle back although the axle back will have minimal horsepower gains it is more for appearence and sound. If you have the 3.31 gears I would say you will want some 3.73's or 4.10's next depending on what your plans are for the car. Pulleys, maybe charge motion plates, if you have the money long tube headers, and you would have a pretty good start.

Also most of your questions can be answered on the first page of the tech forum, go back and read about the first three pages and you will learn alot if you have the time.