Color banding in 24-bit mode

[Grant]

Hello, I'm seeing fairly obvious color banding in X, for example
during video playback and in graphics with a smooth gradient.  I'm
using a 24-bit depth in Xorg.conf and I have the following in
Xorg.0.log:

(--) Depth 24 pixmap format is 32 bpp
...
(==) NVIDIA(0): DPI set to (75, 75); computed from built-in default
(--) Depth 24 pixmap format is 32 bpp

Wikipedia says I shouldn't be seeing banding in 24-bit mode:

http://en.wikipedia.org/wiki/Color_banding

What can I do to improve the actual color resolution?  I've tried
32-bit mode but it doesn't seem to be supported.

- Grant