Step One: Intake. If you're looking for a cheap, easy HP boost, get yourself a K&N replacement filter (any auto parts store should have them, and have a book to tell you which to buy). If you want something more potent, Draxas is working on a full-on custom Cold Air Intake system. Don't know the cost yet, but they're working on a Duratec version before the Vulcan version. Look for the CAI thread started by Draxas_VP
Other than that, you can go numerous directions. I can recommend redoing your exhaust system. Get a high-flow Catalytic Converter (or just remove it if you can legally and morally go that route), larger pipes with as straight a flow as possible. Go with no muffler/resonator for the most power, but that obviously will be loud as hell. The right resonator/muffler combo won't kill the boost much at all. People around here tend to go for the Cherry Bomb resonator with or without a Summit Turbo muffler.
If you're really adventurous, go for a true-dual exhaust system, but that's pricey and has to be done right.
There's also performance chipsets. I still don't know WHICH chip is best for the Taurus, but they sound like a good idea. A new chip for the onboard computer will re-tune your ignition and transmission and whatnot for power instead of fuel economy. I've heard, though, that some chips will increase power AND economy. How that's done is beyond me, must be the way they tweak the air/fuel mixture or something.
By far, though, the best way to add an easy 40 HP to your car is to tint the windows and get a pearlescent paint job. It'll make it go faster, really.
Pax,
Matthew