Ask HN: Realtime video encoding: how to?

I am thinking over an idea of adding realtime video streaming to an application. I.e. I want a desktop user working with an application to be able to start transmitting a video stream of what he's doing (whole desktop or window) to other interested users.

What kind of CPU power and bandwith will be needed for this? Or, what qind of quality is possible with todays CPU power?

What are the obstacles to doing this that you can see?

Are there any applications that do this already?