Heat oven to 400º. Grease rectangular baking dish, 11x7x1 1/2 inches.
Peel baking potatoes and sweet potatoes; cut into 1/4-inch slices. Spread potatoes in baking dish. Drizzle with margarine, and sprinkle with salt; stir to coat.
Cover and bake about 45 minutes or until potatoes are tender.
Mix remaining ingredients. Spoon sour cream mixture over potatoes. Bake uncovered 5 minutes.