what mods to an 04 civic EX

I'm leaving behind a 600+ HP automobile and this is quite a shocker. While I know I'll never get there, I'd like to improve performance WITHOUT making it sound like a go kart. It is important to note that I WILL NOT do anything that will screw up my warranty...

What are my options?

First, airbox. Does the stock airbox actually pull air from under the hood? If so, this is a good place to start...

Intake: AEM or Injen? Are either noticeable improvments? Is water an issue for the "cold air extenders" or whatever they are?

Any value in a throttle body?

I'm not doing catbacks or cat mods. I don't want a big pipe sticking out of the back. What will it sound like with headers and stock exhaust system? Waste of time and money?

Maybe a discrete nitrous setup? What will the stocker take without having to modify the computer setup? 35 shot? Dry? Wet?

What else?

thanks in advance...

04Mag