Rinse turkey, and pat dry. Gently loosen turkey breast skin, and insert pieces of butter between the skin and breast. Place apples inside the turkey's cavity. Sprinkle with garlic powder, salt, and pepper. Place turkey in a roasting bag, and pour Champagne over the inside and outside of the bird. Close bag, and place turkey in a roasting pan.
Bake turkey 3 to 3 1/2 hours in the preheated oven, or until the internal temperature is 180 degrees F (85 degrees C) when measured in the meatiest part of the thigh. Remove turkey from bag, and let stand for at least 20 minutes before carving.