Problems with colour depth and resolution scaling

[Stigmata]

I'm trying to run widescreen resolutions on my 5:4 monitor (1280x1024). It works fine for 720p, 60Hz, letterboxes correctly, 32-bit colour, etc. This also works for 1280x960. But 1280x800 (16:10) only outputs in 16-bit colour, which frankly looks god-awful. I'd like to be able to game in 16:10, but the colour banding is impossible to get over.

Also, it was working fine the other night, and nothing hardware/software-wise has changed.

Specs: GTX460 1GB, Win7 32-bit, Nvidia driver 266.58, monitor is LG1952TX or something.