A crack? YIKES! As I read your question, the first question that came to my mind was, "What caused the crack in the first place?" Before proceeding with repair options, it sounds like you might need to call in a professional to make sure your home is structurally sound, especially since it stretches across the width of a room. How keen are you about keeping the popcorn texture? I know this sounds messy, but this just might be a good chance to get rid of it all together. Newer homes sport the scraped ceiling look, so it might be worth considering. It's much easier to keep clean, too. Best of luck--and let us know what's causing that huge crack!