PREHEAT oven to 350°F. Mix vegetables and pasta in 2-qt. baking dish sprayed with cooking spray; set aside.
MELT 2 Tbsp. of the margarine in small saucepan on medium-low heat. Stir in flour until well blended; cook and stir 2 to 3 min. or until hot and bubbly. Gradually add milk, stirring until well blended. Add garlic powder and salt; stir. Bring to boil, stirring constantly. Reduce heat to medium-low; cook and stir until thickened. Stir in 1/4 cup of the cheese. Pour over pasta mixture; mix lightly. Set aside. Melt remaining 2 Tbsp. margarine; toss with the combined cracker crumbs and remaining 2 Tbsp. cheese. Sprinkle evenly over casserole.
BAKE 20 to 25 min. or until heated through.