1Place potatoes in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and cook for 15-20 minutes or until tender. Drain and cool.
2Peel and cube potatoes; place in a large bowl. Add dressing and toss to coat. Cover and chill for 2 hours.
3Add eggs, celery, onion, cucumber and green pepper; mix well. In a small bowl, combine mayonnaise, sour cream and horseradish; mix well. Pour over potato mixture; toss to coat. Chill for at least 1 hour. Top with tomatoes.