ha, thought i asked this before
http://www.opengl.org/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=3;t=013827#000001
anyways ive run into the same problem.
with nv3x + FBO is it possible to have a 16bit depth as well as a RGBA texture, i cant seem to get it to work.
It seems that depth buffer is definately 24 bits. I also guess that letting the driver choose the size of the depth buffer might not be a bad choice. But you might have some specific needs here…