Preheat the oven to 350 F. Cook pasta according to package direction, and drain. Melt butter in a large saucepan over medium heat. Add in flour, and cook for a minute to brown.
Pour in milk, white cheddar cheese, 1 cup cheddar cheese, salt, pepper, and garlic powder. Continue to cook over medium heat, while whisking constantly until thickened.
Mix sauce with pasta. Spread the mixture out into a large greased baking dish. Top with the remaining cheddar cheese. Bake for 20-25 minutes.