The only difference is Google sees a dash as a space and an underscore is not. Don't ask me where I got this information I think it was in Matt Cutt's blog but I could be wrong.
So some_stuff would be interpreted as somestuff and some-stuff would be some stuff.
I would not use a double dash as shown in your post.
I hope this is what you meant by your question.
Question 2 depends on your server. But if the SEs ignore the upper case (which has happened to me before) in your urls that could cause a big problem indexing, I would avoid it. Windows servers don't distinguish between the two but just about any other I am aware of will 404.
Dan