In "Hamlet" I believe the ghost or King Hamlet was the cause of the whole tragedy that took place in Denmark. Why must he turn his son into a murderer, why can't he just tell any loyal and trust-worthy soldier to do the job?
There are so many factors that could have led to each and every incident that occurred in the play; we could spend hours debating each one. Yes, the ghost coming along, telling Hamlet he'd been murdered, and prompting him to gain revenge was a sort of catalyst. From that point on, Hamlet puts on his "antic disposition" and goes about wreaking chaos and havoc in the lives of everyone around him, all in the name of enacting revenge.
However, before the ghost even came, Hamlet was incredibly upset with the world, with his mother, with Claudius and with life. He was depressed, mopey, sarcastic and withdrawn. Who's to say he wouldn't have snooped around anyway? And he might have still rejected Ophelia because he was disillusioned with women's affections, and could have ranted against his mother for her "o'er hasty marriage" to his uncle.
So certainly some actions can be contributed to the information the ghost gave him; with other actions, it was his previous mind-set that prompted him to act in the way that he did. The news of the murder probably just intensified those emotions. I think that the ghost was definitely a factor, a catalyst, and a very cool way to introduce the concept of murder.
The ghost appearing to Hamlet is significant. It is the son of the murdered; who better to defend the honor of the king than his own son? He was obviously close to Hamlet, and Hamlet was very upset by his death and disgruntled in many ways: perfect fodder for revenge. The ghost had a chance to tell the guards before Hamlet ever came onto the scene, but didn't; what do some random guards that weren't close to the king care? Just as Laertes was so upset by his father's murder that he plotted Hamlet's demise, so was Hamlet an appropriate choice for the ghost.