I installed a PC GTX570 in my Mac Pro 3,1 some time ago & it was definitely working both with the stock OS X driver & the Nvidia Web Driver. I am now running Mavericks OS X 10.9.5 & realised that it was using the stock driver but when I switch to the Nvidia Web drivers then only one display works. I have two monitors (HP LP3065 & HP LP2465) connected by DVI but cannot get both working at the same time. I have both monitors connected & it displays only on the LP2465 on port 1 (furthest from the motherboard) but if I unplug that one then the display switches to the LP3065 on port 0 (nearest to the main board). It doesn't make any difference if I swap the monitor connections round I still can only ever have one monitor active at a time. It works fine with the stock driver. The GTX570 is in slot 1 & is the only graphics card in the machine. Is this a problem with Mavericks? It was definitely working with Mountain Lion. I was considering upgrading to Yosemite & will definitely do so if it fixes this issue. Although is the Nvidia Web Driver THAT much better than the stock driver? I get to use CUDA with both which is the important thing for me.