Well, your best bet for power is a turbo or supercharger.
Those are really expensive and will probably run you for a couple grand, each. But your power will increase Immensely. You will also need to upgrade the cooling system and performance chip. A full set with a performance chip, cooling upgrade, turbo, all the other little things, and a manual on how to install will cost about $5000. None the less, you'll increase your power by about 56%
Give it a shot, search the internet, find a deal.
Here's a good place to look:
http://www.turbo-kits.com/mustang_turbo_kits.html#99-04mustang
Unfortunately, the newer the car, the more expensive a turbo can be.
Look for more help here:
http://mustangforums.com/
The people in those forums can help a lot more then I can, have fun with your 'stang!