In large bowl, mix sour cream, mayonnaise, milk, lemon juice, tarragon, salt and pepper. Gently stir in potatoes, eggs, celery and olives. Line a 2 1/2 quart bowl with plastic wrap. Spoon salad into bowl, packing well. Cover and refrigerate about 3 hours. To serve, invert on a plate and decorate with tomato wedges.