Sorry to answer a 2 yrs old question, but this might be worth it to clarify other users. As far as I know, you actually can upgrade the processor of this iMac to any other processor which consumes the same power (TDP) as it's predecesor.
For example, the most powerful processor for this iMac is the i7 3770S, which has a TDP of 65W, the same as 3330S and 3470S i5s. Of course there are other i7s with more power than the 3770s, but with a higher TDP (77W, even 130W). You can actually install them on the socket, but the system won't boot up!