That's the scoop - NVIDIA-GLX as reported by 'xdpyinfo' disappears on my machine with versions 5328 and 5336. Versions 4496, 4349 and 2960 all display NVIDIA-GLX.
I also believe that I can see a visual difference in 800x600 mode with 5328 and 5336 - the __GL_SYNC_TO_VBLANK seems to be ignored, or at least not working very well. With both of those drivers in that one mode I can see a lot of image tearing in the gears. This is with the gears running in the default window size.
I don't get that effect with the other versions, as far as I've noticed so far.
So for me, I'm sticking with 4496 for now.
Oh, for final detail, here is my export of the 'vblank' variable:
Just like the instructions said!