Open up the exhaust, 2-1/2" from the manifolds back; get a tuner or "tunes for life" deal and get rid of the stock tune.
Then replace the stock upper plenum, it is the weak point in the intake tract. 70 mm TB too though it's not as important as swapping out the plenum--more about that here
Lower ratio (numerically higher) final drive gears will make the car pull stronger, 3.73:1 and 4.10:1 are the most popular picks. The tuner/tunes will let you correct speedometer for the new gears.
After that there's not a lot of "bang-for-the-buck" stuff to be bolted on. A lighter flywheel and UDPs will liven up hard acceleration, as will an aluminum driveshaft. However if you are not the sort to accelerate briskly you will never know they are there.
Aftermarket CAIs are a waste of money, the stock intake is a CAI and a pretty good one at that...
- 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum, Delta Force tuned,
Steeda UDPs, Ralco flywheel, RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.
New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq