What price range are you looking at? Also, what power supply do you have?
Based on your requirements and current specifications, I would go for a Radeon HD 7770. The Radeon HD 7770 will run WoW very smoothly, and it will probably run NWO as well. It will also run any other game you wish to play as well, but not maxed out. It costs just over $100, but performs very well for its price.