The cheapest way to increase horsepower is to remove the intake silencer. It's free, except for an hour or so of your time. Search the projects section for information.
After that, it gets expensive fast. The range of mods runs from a $45 intake kit off e-bay to a $4000 supercharger. The Sport Trac engine is really pretty efficient and powerful for it's size. Most mods just add a few horsepower, although they can add up. If you want to add horsepower on a budget, I think I would do the e-bay air intake kit first, then a new y-pipe, then a cat back exhaust. At that point, you will need to get a chip tuner to re-calibrate your engine's computer to take maximum advantage of the new parts.
Smaller tires (lower, not narrower!) will give you better acceleration if they need changing anyway, but it will hurt your mileage and top end power.