Alrighty... *cracks knuckles*
Follow this: GET IT G
Gears do not add any horsepower but increase acceleration at the expense of top end speed. You will go through the gears faster and by doing so get into your power band faster. The general advice is 3.73 gears for 5-speed. 4.10 gears for automatics. Keep in mind the stock gearing is 3.27. Changing gears require the speedometer to be corrected. See T-Lok below for cost.
Exhaust is a key to making more power. Converting the extremely restrictive Y-pipe design to an aftermarket true dual exhaust can add decent power and better sound. The best way to do this is to have a local shop covert your car to true dual exhaust. It can range from $350 to $500 depending on your midpipe and muffler choice. Mufflers are up to you (check out Youtube for lots of samples). Flowmaster 40's or MACs are a good start. Midpipes you have three choices: Divorced (no crossover), H-pipe, and X-pipe. Divorced pipes are nearly identical to an H-pipe in our even firing cars. We technically do not require an crossover to equalize back pressure. The H will give you slightly deeper idle. An X will give you more horsepower (2-3hp more) but will give it a raspier tone.
T-Lok is a limited slip differential. You stock diff is an open one. Essentially only one tire has power going to it. A T-Lok allows both tires to have power under acceleration. You go from a "one wheel wonder" to a "two tire fryer." Gear and a T-Lok are a perfect match and should be installed together. Total cost including new bearings, fluid and install ran me $700.
Intakes do not add too much power but do not cost very much. A cheap CAI from eBay (BOMBZ Racing) or American Muscle will give you a small boost in power and make your exhaust louder. Both cost around $75-80. You engine is a giant air pump. The more air in, the more air out. Another popular mod is to replace the entire upper intake with that of a Windstar van. They are plastic so they get less heat soak. The runners are slightly longer as well. After the conversion you should see decent gains in torque and horsepower. This can be done for as little as $50 if you know where to scrounge parts.
Tunes and tuners are one of the best bangs for your Mustang bucks. They modify your car's computer's programing to make use of higher octane fuels amongst other things. A tune for 91 octane will get you around 10hp more. Hand held tuners also can read trouble codes (check engine lights) and clear them. They can also correct the speedo from new gears and different sized tires. The best part of a tuner is you can change the tune in a matter of minutes. Just plug in and press a few buttons. Most tunes you get via email like the ones on American Muscle are conservative tunes. You will see power gains but, not as much as you would from a custom dyno-tune. Those are where your car gets a special tune after being run on a dyno. These are the best tunes but also the most expensive. Expect to pay $120+ an hour to get a dyno tune. A hand held tuner will cost you around $370. If you purchase one (I suggest an SCT tuner) from American Muscle you get free tunes for life. When ever you change something on your car, be it gears, tire size, exhaust etc, you can have them email you a new tune.
Phew... I hope this helps.