Until they clash, we'll never know. However, it's been implied that she is around Merlin's level on series 4, being able to fight againist him on equal for a short period of time. Merlin and Nimueh are more experienced though.
Answer 2: Well, it is tough to say whether Morgana is already as powerful as both of them are. Nimueh didn't show her powers, at least not her powers in combat as much as Morgana has so far, so we don't exactly know how strong she was. As for Merlin.. Morgana is definitely not as strong as him yet, since he was even able to beat her as an old man. But even though Morgana probably isn't as strong as those two yet, she certainly isn't far behind. 10:41, September 30, 2012 (UTC)