There are 3 'normal' upgrade options for the NA's, in this order :
1. Single CAI ( Cold Air Intake ) from NeedsWings.
2. 74 mm throttle body from eBay, here, or a junk yard ( car-part.com )
3. Eurocharged tune from eurocharged.com - this remaps the PTCM to utilize the above two to greater advantage. expect 25 - 35 more HP, better response, more torque and better economy.