1. Go on ebay and get some GT take-offs, they're cheap and give you the power you want. The magnaflows are much more expensive, and dont provide much of a difference. IF you want better sound, change out the stock mufflers from your take-offs to flowmasters or Warlocks (with the cut-outs, ive heard they sound really good)
2. Cold Air Induction and Underdrive Pulleys- You can make the CAI yourself, or buy one on ebay that's chrome or stainless steel. Stay away from the full priced MAC and other stuff, just a rip-off.
3. 17" Wheels and tires- While they are quite an investment, shop around and you can find some deals. Go to the classified sections of all the mustang bulletin boards you can find, and you're bound to find some deals.
4. C springs and Bullit Shocks/Struts- better stance and handling, lower your car and make it look like DangerDude's
5. Work up from here.... have fun, but don't break the budget