Just finished reading the section about the Rangers in the Appendix. They haven't really had any contact with Gondor since the fall of Arthedain, over a thousand years ago. I think the book makes it real clear that the men of Gondor, with possible exception of sneaky Denethor and his palantir, have completely forgotten that the Rangers are still out there.
When Boromir comes to Rivendell, he's coming to a place that the men of Gondor view as a myth, they've been out of touch with the North for so long.
Then that's how Boromir knew... because Denethor knew what Gandalf was doing with Aragorn. He probably shared it with Boromir. What do you think?