Boil the chicken breasts in a large pot, reserving the water. While the chicken is boiling, melt the butter in a large, deep skillet and saute the onion and pepper until translucent. Add the soup, tomatoes, tomato sauce, and sliced Velveeta cheese to the onion and peppers and heat and stir until everything is warm and then set the mixture aside. When the chicken is done, remove to a plate to cool and bring the water to a boil. Break the spaghetti into 3 or 4 inch pieces and cook according to package directions. While the spaghetti is cooking, cut the chicken into bite sized pieces and add to the soup and cheese mixture in the skillet, stirring to mix well. Strain the spaghetti and rinse it with cool water and then add it to the chicken mixture, stirring to mix well. Turn all into a large baking dish, cover with foil and bake at 300 degrees for one hour. This is a heavy dish, so a light side dish such as a vegetable or salad would be good with this, but really it''s a meal by itself.