If the audience feels that Hamlet is actually sane by the end of Act I, after he has confronted the Ghost and found out the truth of what happened to him, it seems clear that Hamlet's future actions in the play are going to be based around what the Ghost told him to do in order to secure his revenge. Note what the Ghost says to Hamlet:
If thou hast nature in thee, bear it not.
Let not the royal bed of Denmark be
A couch for luxury and damned incest.
But howsoever thou pursuest this act,
Taint not thy mind, nor let thy soul contrive
Against thy mother aught.
Based on these words of the Ghost in Act I scene 5, it seems reasonable to assume that Hamlet, whether sane or not, will spend the rest of the play trying to work out this revenge against Claudius and make sure that his father's ghost can rest in peace. Having had such a powerful confrontation with the Ghost, Hamlet at the end of Act I is clearly very visibly effected by the responsibility placed on him by the Ghost, and it is obvious that this will become the governing theme for the rest of the play as Hamlet works out how to revenge his father's death.