An HD ready set will be perfectly adequate for Sky as it is at the moment, but a Full HD model might be a wise investment if you're considering a move to Blu-ray/HD DVD in the future.
There's also the slightest suggestion that TV might move on to 1080p at some point in the future, but any such service will be back-compatible with 720p/1080i-capable TVs, and one could chase future standards forever. There's already talk in the States of 1440p, which is beyond current Full HD screens.