Diaby's a good player on his day, and gives us something Diame doesn't. I'd stick with Abou, myself, as much as he frustrates the fuck out of me when he's not in the mood. Denilson isn't going to be backing anyone at Arsenal up next season.
So really, when it comes to the likes of Diame and N'Zonzi, we're talking about spending £7-8m on a player who sits somewhere in between Frimpong and Song - for me, we have bigger fish to fry.
And besides, as much as I think he needs to, I've not heard anything from Wenger that suggests he's going to play Song (or A.N.Other defensive midfielder) in a more rigid role next season, meaning that 'cover' for Song will likely remain quite a broad term.