Surfing through the web, I found a cite from the bible on
http://www.bibleview.org/en/modules/Judges/26_Slingshot/
which said:
 
As the Philistine moved closer to attack him, David ran quickly toward the battle line to meet him. 49 Reaching into his bag and taking out a stone, he slung it and struck the Philistine on the forehead. The stone sank into his forehead, and he fell facedown on the ground.
 
50 So David triumphed over the Philistine with a sling and a stone; without a sword in his hand he struck down the Philistine and killed him.
 
51 David ran and stood over him. He took hold of the Philistine’s sword and drew it from the scabbard. After he killed him, he cut off his head with the sword.
 
 
Now why does it say he killed him TWICE, when the shot with the stone would have finished Goliath? What does the second "killed" mean? Does he finish him with the sword, because he was only unconscious? Or does the second refer to the stone shot, which already killed him? Do you have any ideas?

Cutting the head off was a simybal also. The other day on TV I saw a show on giants. When it went into the story of david and golith. It showed david in a church (is he a saint?) in stainded glass with the head of golith.

If the stone had actually killed Goliath, then the beheading could have been symbolic.  David had killed his enemy and takes the head as a trophy.  Or sometimes mutalation of the body is considered to be a dishonor to the fallen or their family.  I geuss it's one of those things that requires a primative mind to comprehend....
 
That's just my 2 cents on the matter.

I've always understood it to mean that David killed Goliath with his sling. David was a big kid, not some scrawny little runt. He talks of killing a bear and a lion with his hands(1sam 17:34-36). Also, King Saul was a big man(1sam 9:24). Several verses say that he is head and shoulders taller than any other israelite. When David goes to his camp, saul looks the boy over and says, Here, wear my armor. David declined, not because it didn't fit, but because he didn't know how to use armor. (1sam 17:38-39)  
 
So here we have a large young man, who has nothing to do all day but sling rocks at anything bothering his father's sheep. When describing his actual shot, it says that the rock sank into Goliath's forehead(1sam 17:49).  
 
I've personaly put rocks through doors and as much as an inch of plywood.  I'd say that Goliath was stone dead when David lifted his head from off his shoulders. The whole beheading thing, I'm sure, was to make sure none of the phillistines mistook what just happend. Bet it put the fear of God in them real quick.