As for the Dunk and Egg story, it is pretty much as Wert says, it's set in Westeros' past by 3 generations and the story is very self-contained and enjoyable on the whole. I think it's rather silly not to read it just because you don't want to read anything related to SOIAF until that is done, but hey, choices and decisions vary. But do read the other stories at least, perhaps one a day.