In a 3-qt. saucepan, saute celery and onion in butter for 2-3 minutes. stir in broth; bring to boil. Reduce heat. Add potato flakes; cook and stir for 5-7 minutes. Add milk, cheese, garlic salt and chili powder. Cook and stir until cheese is melted. Just before serving, add sour cream and heat through ( do not boil).