Looking for a monitor for my 2012 model iMac
I have recently added an LG FlatIron IPS234V LED monitor, 1920 x 1080, to my 2011 iMac, that I purchased from NewEgg for about $160.  It connects to the ThunderBolt port through a DVI cable and a MiniDisplayPort to DVI adapter, both from Monoprice, for about $16.

After inserting the MiniDisplayPort adapter in the port, and turning the monitor on, the image appears without any changes to the iMac display.  I am quite pleased with the combo so far, at what seems a reasonable cost. Windows can be dragged from one screen to the other.  

I cannot say how this combo would work with a 2012 iMac.  The additional display space remains active even if the monitor is turned off, until the cable is disconnected from the ThunderBolt port.  This is disruptive for remote control, so I try to disconnect the cable before I leave the system.  

I have seen the blue screens when I change the display resolution on either screen, but not otherwise.