I'm looking to upgrade the graphics card in my 27" iMac Mid 2011, which currently has a AMD Radeon HD 6970M 1024 MB. I have been unable to find out what NVIDIA graphics cards are compatible... I would love to upgrade to something like what they ship in the new iMacs- the NVIDIA GeForce GTX 775M graphics processor with 2GB of GDDR5 memory. Can anyone offer advice as to what is compatible and the best place to purchase? Thanks in advance.