I hope to get a 27' iMac next year but I looked at Apple website for base model which costs $1800 but that is for i5 3.2GHz, is it possible to upgrade to i7 in few months?
Possible? Yes . Worth the pain in the butt to open up the iMac, risk damaging the display, the display cable, etc and then gluing it all back together? Probably not.
The cost difference of base model and the built to order model with i7 is $400. It will cost you at least $300 to do that yourself. It's not worth the time and effort really.
I do not recommend opening up the iMac. As the others have said, it is possible but it would be extremely tough. It's an all in one so it is not meant to be easy to open up and tamper with.
It isn't worth saving a few bucks with that risk. You could break the iMac and that would void any Apple Care warranty on it so you would really be in trouble.
I highly recommend selling it and buying a newer one!
I am going to start with base model with i5 and the total price close to $1968.00 included with AppleCare.
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I just started playing strategy games like The Sim 3, Civ 5, Empire Total War Gold Edition, and SimCity 5 on my 2012 rMBP 15" it runs good but I don't like it runs too hot and afraid it might damage logic board.