There is a small performance hit. Nothing really to worry about. I would say you might see something like 5% - 8% at most for card like the GTX 660 Ti. If you were to watch a 1080p movie, then you can expect a much large drop in performance.
I would assume watching youtube videos at 1080p would also make my performance drop? Kind of contemplating to use a TV Tunner aswell and watch cable on my second monitor. Also, my performance would seriously take a dive here?
Maybe I should have gone for the GTX 670...