The "Customize X Configuration" part knows about 8 bpp, 16 bpp and 32 bpp, but it doesn't know about 24 bpp modes. Maybe it would make sense to support 24 bpp (and 1bpp, 4bpp, for that matter) modes, as well.
Added to list of feature requests.
Assigning to Xconfigurator since its being rewritten.
24bpp and 32bpp are really the same thing. Yes, they are also different.
We're not fixing this in the current Xconfigurator.