Originally Posted by russofris
X does not currently support alpha blending, which is where the extra 8 bits come from. Windows RGB is 8b per channel +8b alpha.
Thank you for your time,
That is partially true. X does not support alphablending in uses such as transparent windows, however it does support 32bit depth. Just that 32bit depth is used just like 24bit depth, and so, some drivers support 32bit ad others stay with 24.
The RENDER extension is capable of doing alpha blended compositing operations. I don't know if it is capable of using RGBA or just handles the transparency separately. Anyway it is capable of doing this with the nvidia drivers and 24bit depth AFAIK.