basic mods that kick hiney:
1) shorty antenna ($20)--stronger than stock, smooth, oem replacement, 14" gets same reception as stock, looks classy and sleek but still factory
2)painted mirror inserts ($75)--great fit, paint is perfect and matches, won't come off but is removable in the future
3)tinted light overlays (~$100 for all lights)--easy install if you take your dandy time and CLEAN first with alcohol, doesn't affect brake light intensity noticeably, "clean" look, removes red and orange from color scheme, cheap alternative to clear corners or new brake lights
4)eibach sway bars ($400)--when combined with new tires and/or lowering springs it will result in a complete transformation of your car's handling. i can pass an m3 on an onramp now (on the outside), but that took a number of goodies together, one major one of which was sways
(search online or go to your shop)
5)tint (~$200)--easiest way to make your car look cooler, and keep it a lil cooler in the process (less light=less heat). check local laws though
6)UD pullies ($250)--can be installed in your driveway with rented puller from autozone and a couple tools from the garage in an hour. easy 10 hp and 10 torque, and engine revs faster. improves mileage and increases life of accessories.
7)CA intake ($200-$400, + tuner)--JLT and C&L are the most popular, with JLT being the cheap and simple design (big pipe and big filter), and C&L being a classy and heavily R&D'd pipe design. the steeda and mrt are also common, though gains usually arent quite as big. the wms intake mounts forward and would work with a ram-air setup though none exist to my knowledge yet. the K&N intake is smaller but does not require a retune, though gains won't be nearly as big without one. a programmer is required to tune for all but the K&N. SCT Excalibrator2 is the most popular, but the Diablosport Predator offers essentially all the same features. SCT is favored by custom dyno-tune shops. the tuners will run $300-$400 most places, but discount "packages" are available. gains with intake alone (if the car will run without a tune) are generally 10-15 hp/tq. with a canned tune from evolution motorsports, bamachips, brenspeed, or others will usually net 20-25 hp/tq gains. with a custom dyno tune (another $300 on average), gains will often be 30-35 hp/tq.
8)gears ($550 installed)--the MOST beneficial mod at the track for the money. gears will not increase power, but they will effectively increase torque at the ground through the process of "torque multiplication". you know how first gear accelerates faster than second at a given speed? gears will give every gear the feeling that it is half a gear less than it actually is. going with too "short" of a gear will reduce your car's top speed, though our stangs are geared for 200mph stock anyways, which it will never reach, in an attempt to improve highway gas mileage. 4.10:1 ratios are the most common for manuals and 3.73:1 is most common for automatics. i personally have 4.30s in mine and love it, even on the street, but my setup will not be everyone's cup of tea. traction will be a problem after changing gear ratios, but is nothing that better tires can't fix for the most part. ask anyone and they will agree that gears were the most "fun" mod to have done. get them done by a reputable shop to decrease chances of having "gear whine", a humming sound that fills the car and increases with speed, and to increase longevity of the gears. quarter mile times often drop anywhere from a few tenths up to or more than half a second (from stock) with just gears.
9)exhaust ($300-$1500)--just changing the mufflers (aka "axle-back") will not gain much if any power, but will improve sound. beyond that, one can swap the factory midpipe with either a catted (more likely to pass smog) or off-road (no catalytic converters) x or h pipe. an x pipe looks like that letter and physically joins both exhaust tubes to "cross paths". an h pipe will have seperate pipes with a really short connector between them ("H" shape). the difference will mainly be in sound between them, and both will gain 10-15 hp and similar torque. beyond that, one can change out the headers with either long tubes or shorties. shorties will give some power (10ish hp/tq) and make the exhaust sound more aggressive. long tubes will gain 20-30 hp/tq and make things really loud ("tear-a$$" when hitting the gas). with a full headers/midpipe/muffs exhaust it is recommended that you get a custom dyno tune to maximize power potential. just mufflers will be a couple hundred bucks, and full long tube setups will be around a thousand, not including installation.
10)new/sticky tires ($400 for two)--traction is by far the weakest link with our cars after a few mods. how are we supposed to put down 300rwhp to measly 235 all-weather tires? get some 275/40/17s on there, which is NOT too wide for our 8in rims, trust me. i have nitto 555r, which i recommend for their good grip and excellent price. MT ET Streets will cost a hair more and grip a bit better, but will not last as long and are less streetable. BFG also makes a street-legal drag radial, as well as KDW tires for longevity and a measurable increase in grip over stock.
11)aluminum driveshaft ($400-$1100 +install)--an aluminum driveshaft will take off 20-25 lbs from the car's static weight, and cut the moment of inertia of the driveshaft by 50% or more, resulting in an additional 10-15rwhp from decrease in parasitic drivetrain loss, and will easily knock one to two tenths off of your 1/4mile ET. When combined with pulleys and racing wheels, one can see up to 30rwhp/rwtq, and significant gains at the track. JPC makes a bolt-in unit, but requires a CV conversion joint. BMR makes an exotic carbon-fiber DS, but some argue that vibration could "unravel" it over time, in addition to its 1100$ pricetag. Others have found a cheaper alternative. taking the DS out of an explorer, and having a shop cut it and fabricate on the stock U-joints, balance it, etc can find you the same gains for $400 or close to it, assuming labor charges aren't rediculous and you supply said components to them. If you take this route, i highly recommend getting front and/or rear driveshaft safety loops in case something goes wrong, so you don't have a piece of metal shooting into the cabin at high speed...plus, you will need them if you break into the 12s anyway.
12)lowering springs--this belongs further up the list, but im tired of typing and someone else can address them.
13)most of this stuff from here on out is no longer basic modifications