After some trial and error I was able to get my GTX-285 with 2GB to be recognized instead of the generic 8MB video 10.12.6 told me it was seeing.
I had to:
1. set InjectNvidia=true - make sure nv_disable=1 is DELETED, you don't want to disable nVidia
2. delete NVDAStartup.kext in System/Libraries/Extensions
3. install NVDAStartup.kext from 10.11 using KextBeast into System/Libraries/Extensions
I've got it running in my test bed with two older monitors. I already know the GTX-285 will work with my larger DELL's.
I"m going to try to add a 3rd monitor using a USB2 EVGA UV39 which I'd been using with my older Mac Pro running 10.11 - unless I find one with a higher resolution and USB3?
I was not successful installing the nVidia drivers for 10.12.6, but ... with this replacement kext that allow 10.12.6 to recognize my card, I might be able to install them - or possibly I failed to follow the steps properly. However, as I'm having zero problems so far, why press my luck?
I'm curious how to test whether the GPU's and all features are enabled? Even if they aren't, I no longer get any artifacts when I drag a large object around the screen.
Note: if you upgrade to 10.12.7 (or whatever follows 10.12.6), you'll need to replace that kext again before booting up. Probably the same for 10.13.x.