Cook spaghetti according to package directions. Drain, rinse with cold water and drain well. Transfer to large bowl.
In medium bowl, whisk together salad dressing, cheese, seeds, salt, paprika, garlic powder and peppers until well blended. Stir in cucumber and onion. Pour mixture over spaghetti and toss lightly to coat evenly. Cover and refrigerate for at least 2 hours or up to 24 hours. Top with tomatoes to serve.