I also have same problem. I have 970 graphics card and I have installed current nivida graphics driver (I think 346..). When I installed first OS, it was showing me my GTX970 card under "about my mac", then I installed new driver, I selected nvidia graphics under the nivida graphics preference but wheni reboot it still shows me the internal driver.
I can able to boot using nv_disable=1 but it is booting using internal driver.
I have dual monitor set up.
Sorry If I have hijack your thread as my problem is similar to yours..