Steyn says in his book that Russia will realize that their best deal is to sell Siberia (natural resources rich) to China (as they did Alaska to the US) because when the Chinese want it, Russia couldn't defend it and may as well get some cash for it.
I seem to remember hearing reports of open warfare being waged in the Soviet Far East against the CHICOMS back in the late 1960s/early 1970s along the Amur River.