RALPH:  Polonius hears Gertrude's cry for help and assumes the worst. He also cries out for help, which lets Hamlet know that someone is behind the curtain.

SARAH:  Hamlet, of course, assumes that it's Claudius. It is rather odd, after all, that Polonius should be in Gertrude's private chambers with her, alone at night. 

RALPH:  Hamlet rushes to the curtain and stabs it, hoping that it is Claudius, and that he's finally able to get his revenge.  This would be doubly good, because if it were Claudius behind the curtain, it would be a great time to kill him, while he's in the middle of doing something deceptive.  

SARAH:  Remember that Hamlet wants to kill Claudius when he's doing something as sinful as possible, so that he'll be even more likely to be sent to Hell when he dies.

RALPH:  Hamlet's line here, "Dead for a ducat" most likely means, "I'll bet a ducat that he's dead." A ducat is a gold coin. But it could also mean that killing him was cheap, or simple and easy to do.

SARAH:  And Polonius, master of the obvious even as he's dying, tells us that he's indeed been killed.