First, install the nVidia web driver based not the following website :
http://create.pro/nvidia_drivers
Remember to install both the CUDA drivers and the graphics drivers. Look for the latest based on the OSX build version.
If you experience black or blank screen upon boot, you have to make a few changes. You have to boot into OSX using nv_disable=1.
Then, you follow the following instructions on :
https://www.tonymacx86.com/threads/black-screen-with-macpro-6-1-or-imac-15-or-imac-17-system-definition.183113/
Look for the name given by OSX (for my Z620, it was SLT2). Use Clover ACPI>DSDT>Patches and patch the graphic name to GFX1.
When you save and reboot, you will have full nVidia graphics acceleration!!
There is a new blank screen problem with MacOS 10.12.5 running with Nvidia Web drivers.
Use NvidiaGraphicsFixup with Lilu in Clover/Kexts/Other to fix.
Solution is here :
https://www.tonymacx86.com/threads/black-screen-with-macpro-6-1-or-imac-15-or-imac-17-system-definition.183113/page-38#post-1506262