Get the CAI and tune first because the exhaust is not for performance just sound. If you like the musclecar sound stay with the H-pipe but if you want the nascar sound go with X-pipes. Way to step up to the big leage with the GT

 

I agree, go CAI and tune first. It'll be a different car! It has been my favorite mod so far.
2. wheels
3. exhaust (off-road h and axle-backs)

As for deciding on exhaust, what I did was listened to as many sound clips and videos as I could find and made my decision based on those. When you're ready to go with axle-backs or full exhaust, ask for videos or sound clips and compare them. Looks had a hand my decision as well as drone so it all depends on your taste.

 

Pony Chick those black wheels in your sig are the black bullets right? Those would look sweet on my black gt.

 

Congrats on the upgrade. Great car! I have the Cervinis side exhaust which is as custom as it gets. Very little drone and sounds like NASCAR at WOT and purrs at idle and cruising speeds. I love it! If you go to http://www.magnaflow.com/04sound/04american.asp#fordit will give you an idea of the sound though much better in person. Hope it helps and welcome to the GT world! J

 

Tires at the very least wheelstoo. Nothing else matters if your rubber sux