8bit pallet is more of a driver thing than a video card thing. There were a few driver version on the Nvidia side of things that got it right a few times (back in the Geforce 3 days)....but lately, it's been broken.
Also if Rivatuner hasn't been updated for the said driver, it most likely won't do anything. It won't have control over the switch. That is most likely what happened. If you say you enabled it and the test says fail.
Anyway 8bit pallet isn't a problem with FF7PC. The TNT patch is really an 8bit pallet bypass patch. The TNT patch doesn't work unless it Fails that test.