I have a Dell Latitude D830 which has a 1920x1200 display and a Nvidia Quadro 140M graphics card. It runs windows XP professional German with SP3. When I have a 2nd monitor attached an I have both screens set to 1920x1200 I can not set 32bit color depth. It always defaults back to 16bit. It worked a while ago but doesn't any more.
This only happens when both monitors are enabled. If I disable either one it lets me set 32bit again.
Also when I set the 1st display to 1920x1080 and the 2nd to 1600x1200 i can enable 32bit on both again.