What does it mean when Lady Macbeth says, "Thou'ldst have, great Glamis"?

In Act I, Scene 5 of Shakespeare's Macbeth, lines 20 and 21, Lady Macbeth says, "Thou'dst have, Great Glamis,/That which cries 'Thus thou must do,' if thou have it,..."  In the speech that these lines are a part of, Lady Macbeth reveals that she knows Macbeth has the necessary ambition to desire the throne of Scotland, but that he may not be evil enough to do what's necessary to achieve it (kill the current king).  In lines 20 and 21 she's saying that Macbeth needs a voice inside of himself that cries, paraphrasing, "You must do what's necessary, if you would have it (the thrown)."  This voice, Lady Macbeth feels, might help Macbeth follow through and do what he wants to do (kill the king and claim the thrown).