Hi AK nimitz.. congrats on your purchase... V6 is a nice car.
Few mustang links where u can find tons of info:
http://www.corral.net (most knowledgable members)
http://www.stangnet.comhttp://www.mustangworld.com http://www.mustangmods.comhttp://www.svtperformance.com (03 cobras)
http://www.blueovalnews.com (99-01 cobras)
http://www.mustangexhaust.com (exhuast combos and sounds)
theres more i just cant remeber
Now before spending cash on modifications, drive around in your car and find out which aspect you want to improve first.
Few mods i would suggest for start:
Short throw shifter (a must, prevents tranny damage, short shifts etc. about $160us (either pro 5.0 or Steeda Tri-ax)
Exhaust: that little 1 exaust looks cheezy i'd suggest replacing it with dual exhaust in the rear. H-pipe will not fit your car, but most muffler shop can custom make 1 for you, or theres something premade for V6s. Then you can just buy a GT catback(mufflers) and put them up. That should add some HP (not much maybe 5hp) but sound will be much better. Also remeber sometimes putting new mid-pipe(h/Y pipe) will remove the cats and make your car REALLY loud. (removing cataltics is another 5hp but you'll need to spend extra $30 to turn the check engine light off

As far as air filters, dont expect noticable perfomance gains, K&N or stock its all good.
If your car came with those 15/14 inch wheels replace them PLZ! ..
http://www.buyfordmotorsport.com good prices and all original ford stuff. But stock GT or cobra rims u can buy real cheap from GT owners since most replace their wheels with something else.
When it comes to superchargers.. if you ever get serious forced induction is the way to go.. 3.8L have been upgraded but they still dont hold up boost very well.. It will work, but have cash on the side for possible engine rebuild down the road.
Someone mentioned gears.i agree. you'll need rear end gears for 7.5in rear end . Stick to Ford motorsport brand ccca $160-170 plus labour $200+. Im not sure what final gear ratio for stock 2003 V6 has, i believe its 3.27. (u can get 3.55,3.73,3.90,4.10are better, RPM will go up @ highway speeds) For me 3.73s are perfect as its still very drivable @ highway speeds (RPM not too high).
Pullies are ok but.. @ idle your battery wont recharge, that means on trafifc light @ idle your AC and all electronics will be running of the battery. Worth about 5-8hp.
Mustangs look like off-road vehicles in stock form, so lowering the car will make it look much batter (wont help handleing much). You can futher improve it with shocks, even go for complete coil-over setup. But thats major $$$
few stang parts retailing places:
http://www.classicdesignconcepts.comhttp://www.steeda.com (very expesnive)
http://www.kennybrown.comhttp://www.gefracing.comhttp://www.buyfordmotorsport.comhttp://www.uprproducts.com (lots of billet dressup stuff)
http://www.maximummotorsports.com/ (suspension stuff)
http://www.griggsracing.com/ (suspension)
griggs and Max are 2 the best stang suspension companies, not cheap but real quality stuff.
Many other things i cant remeber.. pick up muslce mustangs and fast ford magazine for more retailers ( this months issue has a nice review and install of V6 exhaust system) and info i cant think of any right now ... hope it helps.. GOOD luck with your new car and welcome to mustangworld !!!!:cool: