Currently looking to swap my 2012 rMBP base with a 2011 27" iMac plus cash. My plan is to install a larger 3.5" drive with a 2.5" SSD for boot, and upgrade it to OSX Server for Time Capsule backups. The items I am less comfortable with involve the thought of the CPU or GPU. I have heard the CPU can be swapped for other 1155 Sandy Bridge chips with no issue, but have heard mixed reviews regarding a GPU swap - is it possible, and if so, any restrictions? I would be more interested in an Nvidia chip than the offerings from AMD. Anything specific I should be wary of regarding compatibility?