I don't think its right to look at how Carlo played Milan. He did favour a 4-2-3-1, but that shouldn't mean anything here. Good managers will always look to play their formation around the squad that they have.
If we sign Aguero/Pato/Villa, expect to see a 4-4-2. If we don't, which seems unlikely now, then a 4-3-3 will still be on the cards.
In a 4-4-2, we will probably have Drogba and Anelka/Aguero etc. up front. The centre of midfield will have Essien and Lampard with Essien holding to allow Lamps to get forward...something like the Fabregas-Flamini partnership. The rest is obvious.
I really don't see us playing a 4-3-3/4-5-1 if we sign Aguero etc. He'd be in the same situation as Anelka and I'm sure the management are smart enough not to ignore that problem. Even playing a 4-2-3-1 just wastes talent. A 4-3-2-1 formation with Anelka and Aguero playing behind Drogba would work, but we lose width...although Bos and Ashley are probably the best candidates in the world to solve that problem!!