I did some testing using mythfrontend with my FX6200 and various NVIDIA Linux (x86) versions.
All tests use MythTV is 0.21-fixes compiled from SVN with --enable-proc-opt and Linux kernel 2.6.18-6-686 from Debian 4.0. (etch)
Here are the results:
185.18.14: Fails, black screen, 100% CPU
185.18.14 w/"NvAGP" "0": Works with HD playback
173.14.20: (legacy driver) Fails, black screen, 100% CPU
100.14.19: Works with HD playback
96.43.13: (legacy driver #2) Works with HD playback
1.0-8776: Works with HD playback
The 96.43.13 driver is from July 1st, 2009 and is supposed to be for the GeForce 2 & 4 series but it works fine with my FX6200.