Under X the value 32 is not a valid color depth. That said under X a setting of 24 is 32. ;D
From the X docs:
"depth means the number of bits in a pixel that are actually use to determine the pixel colour. 32 is not a valid depth value. Most hardware that uses 32 bits per pixel only uses 24 of them to hold the colour information, which means that the colour depth is 24, not 32."
This information is for using the open source nvidia driver included with XFree86. If you are using the closed source binary only drivers from nvidia this may be different. Try looking at the docs nvidia provides for their drivers. I don't use nvidia based graphics cards. Some one else on the board who uses the binsry nvidia drivers might know the answer.