Intake/plenum/TB and midpipe are the easy ones. Gears would be the best first step, but most wait. Those who waited will tell you afterwards they would have gotten them first.
Altogether, my whole setup ran me about $800. FRPP 4.10's, all new bearings/seals, SCT programmer, and a buddy put them in. Completely changed the car. Pulls much harder 1-2-3 gears. Kinda kills the speed in 4th (110mph approx). No need to downshift from 5th anymore. Still gets over 20mpg hwy, and city mpg IMPROVED over stock.