For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Suzanne Gause
When throwing stones at a target, smooth stones move through the air with greater precision. Being a shepherd like David was, he protected his flock from predators this way I'm sure. So he knew this. He chose five stones. ...One for Goliath and the other four for Goliath's brothers. I don't believe he had any doubt in God whatsoever. Otherwise he would have had a whole bag of smooth stones!
Bruce Morgan
While the stone felled Goliath, he was beheaded with his own sword:
1 Samuel 17:51 (NIV)
David ran and stood over him. He took hold of the Philistine's sword and drew it from the sheath. After he killed him, he cut off his head with the sword.
Jack Gutknecht
This is an interesting question, but I see the reference in 1 Samuel 17:45-47:
"Then said David to the Philistine, Thou comest to me with a sword, and with a spear, and with a shield: but I come to thee in the name of the Lord of hosts, the God of the armies of Israel, whom thou hast defied. This day will the Lord deliver thee into mine hand; and I will smite thee, and take thine head from thee; and I will give the carcases of the host of the Philistines this day unto the fowls of the air, and to the wild beasts of the earth; that all the earth may know that there is a God in Israel. And all this assembly shall know that the Lord saveth not with sword and spear: for the battle is the Lord's, and he will give you into our hands."
...as a better reference than the 1 Samuel 17:37 which S. Michael Houdmann uses to support his statement, "David told Goliath that he (Goliath) came with spear and sword, but his weapon was God the Father (1 Samuel 17:37). He trusted God with all his heart, believing that God would tell him exactly what to do and how. And so He did."
Jeffrey Smith
Extra-Biblical texts state that Goliath had four other brothers who were giants too. One stone per giant. David was prepared to take them all out!