For power, NPI cams, if you do the labor yourself, and if you don't plan on doing the PI swap brakes are good, Tint? Seats? What are you looking at doing?
I think the question to you is whats been done already? Do you have something thats not quite finished, if so take care of it. What areas are you looking at Appearance or Performance mods?
i would go with the pi cams and intake, nitrous is good, but the cams and intake is power you will always have.
The PI swap will give you way better top end, but it's more expensive ect. I like the Crower specs for NPI cams check: VTengines Modularpowerhouse comp powerheads there's others
At the very least you need the PI Intake. If your not very mechanical/technical and the RTV method is too much i'd suggest the Livernois PI Intake swap. That will make the rest of your mods work better also. Then like Dalamar said some Crower NPI cams will add more power.