From what I understand, Apple came out with four different versions of the 2010-2011 IMac in the 21.5" size. This might be the same for 27", but I don't know as I specialize in the 21.5" model. Only one versions can be upgraded to an I7 while the other three will only allow the I5 or I3 upgrade. You can tell which of the four models you have by going to "About," placing your cursor over the area where the serial number is located, clicking a few times till you see a four digit "EMC " model number appear. Depending on that four digit EMC number, you'll know if you can upgrade to the I7 or I5 or if you have to keep the I3. Here's the bad news: of the four models, only one is able to safely take the I7 upgrade. Here are the models and applicable upgrade limits:
Mid 2011 IMac 21.5" 12.1 EMC 2428: I7 possible
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Late 2009 ... ... 10.1 EMC 2308: C2D 8600 3.33GHz (3.33 GHz safely. feel free to f-around and find out if others work, I wouldn't... but that's just me)
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Late 2009 ... ... 10.1 EMC 2308: C2D 8600 3.33GHz
Mid 2010 ... ... 11.2 EMC 2389: I5 860 only (no I7)
From what I understand, Apple came out with four different versions of the 2010-2011 IMac in the 21.5" size. This might be the same for 27", but I don't know as I specialize in the 21.5" model. Only one versions can be upgraded to an I7 while the other three will only allow the I5 or I3 upgrade. You can tell which of the four models you have by going to "About," placing your cursor over the area where the serial number is located, clicking a few times till you see a four digit "EMC " model number appear. Depending on that four digit EMC number, you'll know if you can upgrade to the I7 or I5 or if you have to keep the I3. Here's the bad news: of the four models, only one is able to safely take the I7 upgrade. Here are the models and applicable upgrade limits:
Mid 2011 IMac 21.5" 12.1 EMC 2428: I7 possible
Late 2009 ... ... 10.1 EMC 2308: C2D 8600 3.33GHz (3.33 GHz safely. feel free to f-around and find out if others work, I wouldn't... but that's just me)
Mid 2010 ... ... 11.2 EMC 2389: I5 860 only (no I7)
Late 2011 ... ... 12.1 EMC 2496: I3 2100 only