Well, gears are a popular mod. They don't give you any extra power, but they give you a 'kick in the pants' feel. If you have a manual do some 355's or 373's, and if you have an auto go no lower(numerically) than 373's.
To pull some serious power, you will have to build the engine; or just put on a supercharger lol.
While they won't set the world on fire, little things like new plugs and wires, cap and rotor, and a timing increase can get you some more power.
Then there are the bolt-on's like a cold-air, pulley's, high flow or off-road cataltic pipe, etc. Do a search and you should find more information.
For every FULL revolution in your gearset, you take a 1/2 second off your quarter mile. 2.73 to 3.73 will yield you a little better than .5..so if you are running 15.1 QM time then you will be running 14.6. Now THAT is roughly equivalent to adding 50 HP or bumping your car to 270HP. 4.10's will yield you even more go.
T o be honest..Todays cars yield a two fold problem for our outdated motors..First of all they are doing more with greater technology and less cubic inches and ALSO building BIG a$$ engines. The stock 5.0 is waaaaay outdated in technology. It's putting out 220hp with crappy heads, intake, and choked by emissions (compared to todays cars).
Just to get it on par with v6's today and some v8's you'd better at LEAST h/c/i. If you want to beat the competition, you have to supercharge in ON TOP of that. Then you will be blowing them off the road though, lol. It;s fun when someone thinks they will win and u beat them tho. That's why I wouldn't enjoy a vette or a viper..no surprise, no fun.
But the competition is FIERCE. Look at all the chevy's with 6.x liters coming out churning 360+ to 400 + HP, infinite v6's with 300+ HP, Dodge with 340-400+ HP V8's. And they are all street friendly.
Here's some easy mods that'll gain you some performance for next to nothing:
1. Increase timing advance to 14* at idle.
2. Remove fenderwell air silencer from airbox.
3. Remove mesh screen from MAF sensor housing and grind away the bar in the middle with a dremel.
4. Remove some unnecessary weight: strut tower brace (6lb), smog pump (7lb, use shorter belt), rear diff. dogbone weight (8lb).
5. ASP underdrive crank pulley. Not quite free ($39) but if you remove the smog pump, you're gonna need a new belt anyway so do both at the same time.
6. Hollow out the cats.
Then when you've saved some money you can get started with bolt-on parts. I recommend a 3.73 ring & pinion as the first bolt-on (best bang per buck).