Do you have Nvidia Injection set to True in the config.plist file?
If so, then you may want to reinstall the Nvidia Web Drivers and make sure you boot into the system with the following bootflags until you enable the Web Drivers:
-v GraphicsEnabler=No nv_disable=1
Since you are forced to reboot after installing the Nvidia default drivers, you'll have to put these bootflags in again so you can enable the web drivers from the control panel
Once you've enabled the web drivers and are rebooting, you can remove the bootflags and hopefully you'll get a working display. (As long as you didn't set the BIOS to IGPU only and as long as your monitor and connectors are functioning)
Are you still able to log in or does your system lock up at the login screen?
That OEM GPU is hopeless unfortunately, I have had success with the EVGA GeForce GTX 750 Ti and my hackintosh installation without any performance problems. (It can run games, do 4K video editing in Final Cut Pro even though it can't export H.264 past 1080p unfortunately (that's just software limitations from Apple))
The good news is that you can pick up the same GPU for less than $100 but that's just my recommendation.