"Fair is foul and foul is fair," spoken by the Weird Sisters in the twelfth line of Act I Scene 1, has great dramatic significance in Macbeth. It sets the scene for the entire play, embodying the numerous paradoxes interwoven into the text and displayed in the actions of the various characters, most notably Macbeth himself. At the start of the play, Macbeth is renowned throughout the kingdom as a brave and noble warrior, whose epic feats on the battlefield have earned him near-universal admiration and respect. Yet in the paradoxical world foreshadowed by the witches' prophecy, accepted moral values are unceremoniously turned upside-down. Macbeth becomes a dark shadow of his former self, murdering a king and establishing himself upon his blood-soaked throne as a cruel and ruthless tyrant. Appearances are deceptive, and Macbeth's initial appearance is the most deceptive of all.

Your quote by Duncan is from Act 1, scene 6. A similar quote by Duncan is as follows:  "There's no art to find the mind's construction in the face" (1.4).  He is speaking of the traitorous Thane of Cawdor in whose face he couldn't read betrayal. Yet he doesn't realize how true his statement is and how much it will come to haunt him:  he won't see Macbeth's traitorous plans in his face either.

Duncan also says of the original Thane of Cawdor:  "He was a gentleman on whom I built/An absolute trust" (1.4).  This lets the audience know that Duncan's trust will ironically be betrayed by two Thanes of Cawdor: he fully trusted the first as he will do with Macbeth.

A quote from Macbeth is as follows:  "Stars, hide your fires;/Let not light see my black and deep desires:.." (1.4)  This quote lets the audience know, for the first time, that the witches' prophesy has affected Macbeth so much that he is contemplating a black desire.  This is a small hint to the murder plot that will soon unfold.

One of my personal favorites is by the Second Witch:

Second Witch:
By the pricking of my thumbs,
Something wicked this way comes.
Open, locks,
Whoever knocks!
[Enter MACBETH]

Macbeth enters after this, which indicates that Macbeth has indeed become "something wicked" (Act IV, scene i). The dramatic significance relates to characterization: Macbeth embodies wickedness as conjured by the Witches and has become the emblem of the power of fate.

Shakespeare was such a gifted writer that all of his plays seem to offer a wide array of quotations of dramatic significance. In Macbeth, some of these quotes are found in Macbeth's soliloquies (actor's speech to self or the audience).

Examples of some of Macbeth's more popular quotations are in his "Is this a dagger I see before me…" speech, found in Act Two, scene one, beginning at line 33. It is in this scene that Macbeth is on his way to kill his King and friend, Duncan, and he sees the image of a dagger hovering in front of him, almost leading on his way to Duncan's room.

There is also his "Tomorrow, And Tomorrow" speech at the end of the play in Act Five, scene five, lines 17-28. This is the speech that Macbeth delivers when he discovers that his wife has killed herself. He is beginning to tire of the life he has chosen for himself, as seen in his comment that Lady Macbeth would have died at some point.

The previous quotes are of dramatic significance.

There are other quotes that are perhaps not as often quoted, but are still familiar to students of Shakespeare, and also offer dramatic significance. The following quote is found in Act One. It speaks of the Thane of Cawdor and how bravely he goes to his death—dying more nobly than he lived:

Nothing in his life

became him like the leaving of it. 
(I.iv.7-8)

This quote is delivered by Malcolm to his father, Duncan, regarding how valiantly the Thane of Cawdor—traitor to the Scottish crown— went to his execution. (This scene also foreshadows the end of the play, in the death of Macbeth—the new Thane of Cawdor, also a traitor.)

Another significant quote is delivered by the Doctor in Act Five, speaking of Lady Macbeth's sleepwalking (and "talking") malady. However, it is an important theme with regard to who the best healer is for one whose heart and/or conscience is ailing:

The patient must minister to himself. 
(V.iii.45-46)

The Doctor reports that if Lady Macbeth is to get well, she needs to do it herself. He doesn't have any medicine that will heal her heart of her deep sadness and guilt. This could also apply to Macbeth who has destroyed himself for the love of the crown—not the love of his country. The statement is almost prophetic in that the Doctor says what must be done, but we get the sense that Lady Macbeth—the determined and frightening instigator of Duncan's murder—has gone beyond a place where she change her situation, inferring that no healing is possible.

The previous quote is also reminiscent of the biblical verse found in Luke 4:23 found below.

Physician, heal thyself.

Dramatic significance may apply to many different dramatic developments including to quotes that are important to plot development, or to quotes that show character development, or even to quotes that explain offstage events.

These are three of the more important quotes, in my opinion, that show dramatic significance.

Act 1.3 Line 38 - Macbeth: "So fair and foul a day I have not seen." Sets the theme of appearance versus reality and foreshadows upcoming paradoxes.

Act 5.1 Line 39 - Lady Macbeth: "Yet who would have thought the old man to have had so much blood in him?" Foreshadows Lady Macbeth's mental breakdown because of the weight of her guilt.

Act 5.5 Lines 23-26: Macbeth: "Out, out, brief candle! Life's but a walking shadow, a poor player, That struts and fret his hour upon the stage, and then is heard no more." Develops the themes of the futility and the brevity of frail life.

But, of course, there are MANY more important, dramatically significant quotes throughout the play.

There is a good description of dramatic significance supplied by the Dufferin-Peel Catholic District School Board in Mississauga, Ontario, Canada.