In large saucepan, bring 6 cups lightly salted water to a boil. Add potatoes. Return to a boil. Reduce heat; simmer 5 minutes. Add green beans; cook an additional 8 to 12 minutes or until potatoes and beans are crisp-tender.
In small bowl, combine yogurt, salad dressing, horseradish, salt and pepper; blend well. Set aside.
Drain potatoes and green beans; rinse with cold water. Drain well. Place in large serving bowl. Add chicken and celery. Pour dressing over salad; toss gently to coat. Line 4 individual plates with salad greens. Spoon salad onto lettuce.