Guys. I've got a late 2012 27'' iMac with GeForce GTX 680 MX. Do you think it is possible to upgrade to NVIDIA GeForce GTX 780M with 4GB of GDDR5 (available in the newest late 2013 iMacs)? Does Apple handle such upgrades? BTW- I just hate when they do a 'refresh' a year later... THANKS VERY MUCH for all the replies.