As Cathy says, but a narc can also mean a drug squad cop, undercover or otherwise, as well as an informer. There used to be a word "Nark," too, which, in this part of the world at least, meant anyone who was an unwelcome pain in the arse. Too easily confused with "Narc" to be used much in conversation these days.
Sixpence the richer means what it sounds like — financially a bit better off, but not enough to brag about. If you come away from a card game sixpence the richer, for instance, you didn't make much, but at least you didn't lose.