2006 Mustang returns with its popular retro redesign. Differences over the previous year include: choice of Standard, Deluxe, and Premium series options; optional 18-inch wheels for the GT
The GT model adds a V8 engine, anti-lock brakes, dual exhausts, fog lamps in the grille, rear decklid spoiler, 17-inch wheels, and a 6-way power adjustable driver's seat. Both models can be upgraded to the Deluxe option which includes the Shaker 500 Audio system and a seat upgrade. 2006 also sees the return of the Mustang Shelby as an upgrade GT for Hertz, the GT-H.
http://mustangattitude.com/mustang/2006mustang.shtml
From wikipedia: The Mustang GT builds on the base Mustang's standard features. In addition to its higher performance powertrain, the Mustang GT features a stiffer, better handling version of the standard suspension, larger 12.4-inch front brake discs (versus the 11.4-inch discs used on V6 Mustangs), standard four-channel ABS with traction control, a two-piece drive shaft, a stainless steel dual exhaust, standard grille-mounted fog lights, and 17-inch wheels with optional 18-inch wheels.