Cook, rinse and drain pasta shells. Cut cream cheese into cubes. Cook beef in a large skillet over medium high heat until brown, stirring to separate meat; drain fat. Reduce heat to medium low. Add cream cheese,, salt and chili powder; simmer 5 minutes. Toss shells with butter; fill with beef mixture. Arrange shells in buttered 13x9" pan. Pour taco sauce over each shell. Cover with foil. Bake in preheated 350~ oven 15 minutes. Uncover; top with cheddar cheese, monterey jack cheese and chips. Bake 15 minutes more or until bubbly. Top with sour cream and onions. Garnish with lettuce.