The fact that you think a defender can become world class overnight is ridiculous in itself. Eboue may have gotten much much better after the African Cup of Nations but you definitely cannot say that Toure became the player he is only after the ACN. If you had watched all of Arsenal's games this season you would've seen that he together with Jens was keeping us together at times and that our horrible form during the ACN was obviously largely due to him missing. I don't see how you can compare Toure who's playing with a young, inexperienced, make-shift defence with players such as Nesta and Cannavaro who are playing with some of the world's best defenders and best goalkeepers (Dida's pretty shit this year but still). I didn't see Nesta keeping Milan together this year when teams like Parma (I think) put 3 goals past them. By the way, I didn't say Toure is better than Nesta, I'm just saying that you really can't say Nesta is better because of where Milan is in the table, how many goals they've conceded nor can you say that he's better than Toure at all until they play in a relatively equal quality defensive line. I do think that Toure can be ranked up there with the world's best and I'd say that one current form he is the best at the moment (Terry's made some pretty silly mistakes this year which have gone unnoticed at times, Milan's defence was again, shit and Rio Ferdinand who used to be ranked up there with them has had some absolutely atrocious performances for Manu this year). Also, I'd say that an Arsenal fan, biased as one may be, is in a much better position to judge Kolo Toure as a defender than you are seeing how we watch him every game (I watched every Ivory Coast game bar one in the ACN as well).
Also, I remember being laughed at by Juve fans when I dared compare Eboue to the great Zebina, even while Eboue was in amazing form and Zebina had only played like 4 (?) games the whole season.