The base Mustang comes with a 4.0L V6 with 210 hp and 240 lbs. of torque.
The Mustang GT is slightly more expensive and has a high performance 4.6L V8 producing 315 hp and 325 lbs. of torque. For 2011 Ford will scrap that engine and give it the 5.0L V8 giving it over 400 hp and 400 lbs. of torque, an 85 hp difference which will be a lot faster.
The Mustang GT500 Shelby is the Big Bertha. It features a 5.4L Supercharged V8 producing over 540 hp and 510 lbs. of torque.
So yes the Shelby is the fastest, most powerful, and expensive model of the Mustang. The Mustang GT is second, then the V6 is the least fastest, least powerful, and the cheapest.
Picture comparison:
V6- http://img.worldcarfans.com/2008/11/large/2010-ford-mustang_63.jpg
http://blog.caranddriver.com/wp-content/uploads/2009/09/2010-Ford-Mustang-V-6-10-440x268.jpg
GT- http://image.motortrend.com/f/15324383/112_0903_16z+2010_american_muscle_car_comparison+2010_ford_mustang_gt_front_motion.jpg
http://www.edmunds.com/media/il/wallpaper/033009/10.ford.mustang.gt.act.f34.2.1600.jpg
You can tell the difference from the blacked-out grill, two big fog lights of the grill, and an aggressive lower/bumper grill.
GT500- http://autoworld.files.wordpress.com/2009/01/ford-mustang-shelby-gt500-2010-img_1.jpg
http://pulseautosport.com/wp-content/gallery/2010-ford-mustang-shelby-gt500-convertible/ford-mustang_shelby_gt500_convertible_2010_1280x960_wallpaper_01.jpg
They usually have racing stripes, features a Cobra insignia on the grill, side, and back, and a GT500 stripe on the lower side of the car.