I have a cMP with a vanilla GT120 card. Since I essentially do photo stuff (Lightroom 99% of the time), this card served me well, including on a 30" display. But there are times where I would maybe like to test OpenCL or benefit from some boost in some occasional rendering (video is a very occasional hobby). Can I just drop any GPU in the second slot, and keep the display on the GT120? Can I mix and match AMD and nVidia? An AMD R9 270x looks like a good deal for instance.