I had an '05 GTO back when they were new, and I had modified it with JBA (50 state legal, as I live in California) headers and a Corsa Sport exhaust. The car sounded great and was a blast to drive. Future mods for you could easily include supercharging or an upgraded cam and heads. Drawbacks: Dated styling, and wheels/tires that are a bit too narrow to effectively hook when launching the car.
The '04 has a 350hp 5.7L LS1 engine with an inefficient dual exhaust that exits out one side of the car, while the '05-6 has a 400hp 6.0L LS2 with a true dual exhaust, along with a redesigned and more interesting looking hood.