Nitrous is a decent way to give quick HP and is probably gives the best bang/buck ratio. But it's not legal in many jurisdictions, can be hard on the motor, can be finicky (re purging, backfires, leaky solenoids etc etc) but if you stay sane on the shot, it might be what you're looking for.
A 100 shot is a good, safe compliment to a 4.6 car. It is risky, but keeping up on solenoid maintenance and running with a WOT switch and window switch help out a lot.
Fit my needs, it's a blast, and I've ruffled some feathers in the process, haha.
Its an automatic and i drive it daily. the only thing different on it is
flowmaster mufflers. At first i was thinking CAI and TB with a tuner and O/R h-pipe. I read on forums they don't give you as much as they are stated to give you unless you have other parts with them.
Another thing on this - manufactures claims of HP gains are almost always inflated. You won't gain 15hp from a cold air intake, 20hp from an x-pipe, etc. They always test parts under "optimal" conditions (as in built motor, other parts already on the car, etc.).
The honest truth is bolt ons, while they do offer gains (although small), are good money wasters except for a select few.
Since your car is an automatic, there's really no better bang for the buck than a gear change (4.10's are the general consensus). If you want more out of it, search for the "j-mod", really a great mod for the auto's in our cars. Even more? Get a higher stall converter and enjoy the fun. Buy accordingly though, it's not a one size fits all deal. Figure out what you want from the car and go from there.
On top of that, switching to a catless h or x pipe will compliment the sound of your Flowmasters, and will uncork the engine a little bit.
With more air going out, an air intake will help get more in. No need to go overboard here. JLT seems to make the most power though.
Honestly a killer 2v setup for an auto car is this:
Gear swap
Stall
J-mod
Exhaust
Nitrous (100 shot is fairly safe, offers nice gains)