Try going to http://rpmoutlet.com as they have some great combinations of add-ons which they have tested, and as another poster said, the air intake, and exhaust are the cheapest upgrades. You can add about 50-60 HP with an air intake, a larger throttle body and elbow and even more with some headers. You can spend about $800 total on the Intake and a few hundred on the headers and take a 300 HP engine to somewhere between 360-400 HP. Of course , if you want to spent a few thousand for supercharger, you can end up in the 400-500 HP range.
power control module is a good way to add hp. it reprograms your computer and sends the correct map to the computer for timing and ignition. the cost is anywhere from $400 to $550. hypertech.com have them with links to other companies as well. the pcm first loads the stock settings and saves them,then ask a series of questions such as tire height,shift point etc. the reason it saves the stock settings is if you want to return to stock you can do so in a matter of a few minutes. i've read claims of 50+hp gains depending on the engine size. contrary to claims of voiding warrantees is false,however if your car dealer found out they may void any extra warrantees off offered by them. HAPPY MOTORING...
air in air out... exhaust and induction are the quickest, cheapest ways to make power with out f-ing up the warranty .... try anderson ford motorsports...its a ford dealer with an extensive history in mustang drag racing
Your Ford dealer would be a good place to start. Ford racing has a bunch of performance parts available. As another has said, you can reprogram the cars computer to gain more HP.
There are a bunch of mustang enthusiasts out there and a bunch of mustang clubs. Several high performance mustang magazines are out there too. Checking out the forums on mustangs will let you chat with other mechanics and owners and you can benefit from their knowledge.
go to http://www.performancecenter.com/ they have the guaranteed lowest prices. I bought my intake, exhaust, and throttle body spacer from there. A friend of the family owns a customizing shop and the prices from this website were cheaper than the prices he pays.