Cook rice mix in a large nonstick skillet according to package directions, using 1 tablespoon margarine and 2 1/4 cups hot water. Remove mixture from skillet; set aside. Wipe skillet with a paper towel.
Coat the skillet with cooking spray, and place over high heat until hot. Add chicken, mushrooms, and garlic powder; sauté 6 minutes or until chicken loses its pink color. Combine rice mixture, chicken mixture, sour cream, pepper, and soup in a bowl; stir well. Spoon mixture into a 2-quart casserole coated with cooking spray. Combine cracker crumbs, margarine, and poppy seeds; stir well, and sprinkle over chicken mixture. Bake at 350° for 35 minutes or until thoroughly heated.
Make-Ahead Tips: You can assemble the casserole ahead of time, omitting the cracker-crumb mixture; cover and chill in the refrigerator or freeze (thaw frozen casserole overnight in refrigerator). Let stand at room temperature 30 minutes; top with cracker-crumb mixture, and bake as directed.