In Macbeth, what is significant about the differnce between the events that open the banquet in Act III and those that end it?

The simplest answer to your question is that the opening
of Act III, scene iv is full of joy and celebration for Macbeth's being named Duncan's
successor.  The thanes have gathered to honor Macbeth and are very deferential to him in
recognition of his new title, King.  However, once he, as far as they can see, begins to
act crazy, the thanes grow suspicious and are shooed away by Lady Macbeth.  So, the
atmosphere does a 180 degree, which is significant because of the suspicion about
Macbeth that is born by the end in the thanes who have witness his odd
behaviour.

Let's look at the events in more detail.  All
seems appropriate and as it should be at the opening, as Macbeth welcomes the thanes and
offers them their seats.  He is then interrupted by a visit from Banquo's murderer, with
whom he confers, confirming that Banquo is, indeed, killed, but learning that Fleance,
his son, escaped.

Once the murderer has left and Macbeth is
invited to sit in the chair reserved for the King, the scene is immediately altered for
Macbeth.  He sees the ghost of Banquo sitting in his chair.  He begins to converse with
the ghost, while all the others in the room see only an empty chair.  He
says:

. . .If
thou canst nod, speak too.

If charnel-houses and our graves
must send

Those we bury back, our
monuments

Shall be the maws of
kites.

. . .If I stand here, I saw him. . .
.

Blood hath been shed ere now. .
.

And since too, murders have been
perform'd

Too terrible for the ear. .
.

And with his mention of
"charnel-houses, "blood" being "shed," and "murders. . .too terrible for the ear,"
Macbeth has sealed a suspicion in the hearts of all the thanes who have witnessed his
speaking about them.

Lady Macbeth makes feeble attempts to
cover Macbeth's words and actions when the ghost appears the second time, but the damage
is done.  The thanes know that Duncan was murdered and they will learn soon of Banquo's
fate.  The words and actions of Macbeth condemn him in their eyes and, for the balance
of the play, he will have no real friend or ally, all are suspicious and turn against
him.  By the time that Lady Macbeth has cleared the banquet hall, the damage has been
done and Macbeth is suspected as a traitor and
murderer.

This scene is the point of no return for Macbeth
and the end of any emotion, whether fear or love, for him as well.  He becomes a killing
machine, interested only in keeping the power that he has
gained.