First of all, i suggest the 2-cylinder bolt-on kit. This kit can be found on just about any major mustang performance website.
Next, i suggest the Tornado. This baby makes your intake air enter the intake a lot faster so you can get more power. Check Ebay for this amazing performance upgrade.
Thirdly, i suggest wheels and tires. All of the power in the world is useless without the right meats to apply it to the pavement. I suggest going to one of our vendors and ordering a wheel and tire kit. Wheels are generally the same. Just be sure to get some that are manufactured in china from random metallic alloys smelted by 12 year old chinese children. As far as the tires go, you cannot go wrong with either Nankang or Sumitomo. Those are what i run on my beast.
Furthermore, once you have peaked at the pinnacle of performance a "tough" pony is not complete with out the most exuberant body kit money can buy. I suggest trolling ebay for sweet deals. You can find body kits specifically made for the S197, however shipping may take a while. The chinese are very slow at shipping to the US these days.
Once again, welcome to the forum.

See you on the streets!