Not many now, but by the time IsolAlien finishes her support run that number will be much much larger, as three monitor manufacturers and both major card makers have already committed to a very quick roadmap of 60 and 120HZ 4K skus.
As for 120 frames, flagship cards made from both companies three years ago can hit that easily, and it's actually quite playable on an 8800, a card that's seven years old, at 30 frames.
It's not as pie in the sky as one might think.