Place potatoes in a Dutch oven and cover with water. Bring to a boil. Reduce heat; cover and cook for 10-15 minutes or until tender.
Meanwhile, in a large skillet, saute celery and onion in butter until tender. Remove from the heat.
In a large bowl, combine the stuffing cubes, poultry seasoning and sage. Stir in broth and celery mixture. Transfer to a greased 13-in. x 9-in. baking dish.
In another large bowl, combine the turkey, soup, garlic powder and 1/4 cup sour cream; spoon over stuffing mixture. Drain potatoes; mash in a large bowl. Beat in the cream cheese, pepper, salt and remaining sour cream; spread over turkey mixture. Sprinkle with cheese.
Bake, uncovered, at 350° for 30-35 minutes or until heated through