I would pick Angelina Jolie to play Lady Macbeth, she has played that type of character in many movies before, she is strong and plays a good temptress/gets what she wants kinda girl.
The casting for Macbeth is a little harder... And i hate to sound unoriginal but I'd say Brad pit would be the best actor for the job, for one Angelina and Brad have great chemistry in the movies they star in together, two Brad Pit was an awesome Achilles in the movie Troy and i believe there are many of similarities between Macbeth and Achilles.