I thought narcissism was a sub-genre of sociopathy.
I just can’t see this as “jerkish” behavior. Farrah walks onto his show incoherent and apparently drunk or stoned, and he tries to keep the discussion going. Seriously, I’ve seen Farrah interviews before, and she’s just not that stupid and vacuous. I’m not sure what else he could have done other than throw it to commercial and bring the next guest in early.
Similarly, the Kinski thing. It would have been hard for him to continue trying to do a serious interview when the whole audience was laughing at her, and she’s pretending she doesn’t remember the snake picture, her hair is normal, and she’s not on some kind of drugs.
Dave has an edge, as others have mentioned, but I don’t see him as being “mean” to his guests.
The firehose thing, on the other hand, is definitely a jerkish thing to do.
No. Sociopathy is not caring about others. Narcissism is about thinking everyone loves you. These have no relationship.
It’s not a firehose! It’s a garden hose. Does that make a difference? 
Oh. That’s fine then. Carry on.
Whew! Glad we settled that.
Both are about not caring about others. The reasons are just different. A Narcissist cares about himself and his desire for attention so much that other people’s feelings often get left out. A sociopath, on the other hand, has taken his ability not to care about others to an extreme, and actually enjoys causing pain. He just doesn’t understand why causing people pain is wrong.
There is a lot of debate over whether they are related or not. I find that the WikiAnswer to this questionis quite informative (if poorly formatted), especially with its external links.
ETA: As for Letterman, I think he comes off as a narcissist who is honestly trying to be better. He has the mannerisms of a jerk, but seems not to do anything (in public, anyways) that quite crosses the line. The thing is, that still fits perfectly with being a narcissist: he doesn’t want the negative attention for being a jerk.
So when you were 21-ish, and one of your friends got blasted at the bar, you would have everyone sit there and listen to him blather on about nothing (if that much), or would you tell him to STFU and shove him in a cab?