For the enthusiast, this is what's known as "a good start".
The aftermarket has embraced this new motor, thanks in part to a technology sharing program from Ford, and a vast array of performance parts are quickly becoming available. One of the first upgrades many owners will install is a cold air intake - a device that brings cool, dense outside air down a straight path into the motor. It's a relatively inexpensive way to boost power and potentially gas mileage, too.
Two Schools of Thought
For modern cars there are two strategies for cold air intake design. The first is to build one that flows a little better than the stock intake system but still operates within the specs of the factory engine management computer. This results in a lower cost solution, but offers less performance improvement than can be had through the second method.
The other strategy utlimately gives more power and greater flexibility, but at a higher price: create a superior intake system and reprogram the computer to deal with its new airflow characteristics. This video from AmericanMuscle.com shows the obvious visual differences between the two types of intakes.
If the lower cost solution is enough, there are a half-dozen products available from a half-dozen manufacturers that will do the job about equally well. There are many options as far as finish and design, but any kit that isolates the filter from warm engine-compartment air (without sitting too low to the ground where it might suck in spray from wet roads), and sends it down a relatively short, straight, smooth tube will do the trick. These kits on the 5.0 typically provide 10-12 more horsepower and similar torque increases.
If you're willing to spend almost triple the price, you can get triple the power (or more), plus end up with a tool that will come in handy if you plan other performance upgrades in the future. For 2001-generation Mustangs, the cold air performance king is the Steeda intake system. In addition to the intake you'll need an SCT flash programmer with a custom tune supplied by Steeda. This setup will eke the most power possible out of your new intake - to the tune of 44 horsepower and 52 foot-pounds of torque!
Whichever type of system your budget allows, you'll enjoy increased power from an easy to install kit with a reusable filter. You might even see slightly better gas mileage, if the newfound power doesn't compel you to keep your right foot a little closer to the floor.
Remember, street racing is stupid - enjoy your ride, but drive safely!