Preheat oven to 400 degrees. Prepare spaghetti according to package directions. At the same time, brown hamburger. Add onion and pepper to browned meat. Cook until soft. Drain meat mixture. Add pasta sauce to meat and simmer while making other layers. Combine cream cheese, sour cream, parmesan and garlic salt. When all of the layers are prepared, put spaghetti in the bottom of a greased 9x13 baking dish. Spread cream cheese mixture over the spaghetti. Pour meat sauce evenly over cream cheese layer. Sprinkle the grated cheese over the top. Bake in preheated oven for 30 minutes or until top is lightly browned and bubbly.