Personally mate, as a pure central midfielder I think it is very easy to improve on Nolan. Obviously in terms of his leadership and goal output it makes him useful, but really you could sign hundreds of midfielders with more guile, technique, athleticism etc.
Barton is slightly different because I do think he's a genuinely good footballer.
In a nut shell.
I would hope that Cabaye is an improvement on Nolan when it comes to general play, however I don't expect him to score the goals Nolan did. That's why I'm hoping for at the very fucking least for a striker that will massively improve on our forward options. With that, and extra possession, improved passing, technique etc that Cabaye is supposed to give us over Nolan, plus the return of Ben Arfa, that should mean Nolans goals won't be missed as much.
It's daft to say that Gosling will be an improvement on either though.