i dont think itll add hp but im sure the torque ratings will increase and itll definitly increase acceleration to feal like more hp, but do a gear swap id say 355s for autos and 373s to 410s for 5spd also try sentetic lubes like oil, rear end juice, manual and auto trans uses semi syn from FORD
cold air kit, a cat-back system, 70mm throttle body if you dont plan on going too crazy with mods, or other wise 75mm, upper plenum, if you do alot of interstate driving get 3.55 gears or if you do more around town stop and go traffic do 3.73's (makes it perform better, not a pwer adder), im not a big fan of nitrous on stock motor (always heard not too good for stock plastic intake manifold.), programmer, underdrive pullys, intake manifold, heads stage 1 or 2 depends on what you want, cams. something to keep in mind though is to make sure your car is ready for what you want to do. say if you want to add a blower-you need to probly mod the rear end, suspension and deffinately brakes.
Actually, if you want to be technical, things that add power are literally called power adders. Superchargers, Turbo's and Nitrous. Now if you want to "free" up more horsepower, then there is a slew of possibilities.
If you really want to stay in the bolt on game, do yourself a favor and don't limit it to intake/exhaust with some pullies and a hand held tuner of some sort. There's -so- many other things you could do to make your car faster or perform better.