I'm wondering how much is scripted. When hes on the subway or in the street are those people running away from him or yelling at him, are they actors?

Very Nice.

It's hard to tell a lot of the time. I'm not sure about the scenes you're referring to, but I think that ones like the dinner party, the rodeo, and the drunk frat boys were all non-scripted (which is incredibly frightening, if you think about it).

That's one of the funniest movies I've ever seen.

Yeah, that was the one thing I didn't like about this film. The un-scripted stuff was hilarious, but I constantly found myself questioning whether or not a scene had been scripted and wasn't real (which I'm sure many weren't) and when it obviously was scripted I didn't find it all that funny.
Overall, a hilarious film though. If you want to see more of him at his best just type "Borat" into youtube.com and get ready to laugh.