Ok, Alien_Hominid, I got it working. The log is being created now too...
X Window System Version 6.9.0
Release Date: 21 December 2005
X Protocol Version 11, Revision 0, Release 6.9
I had removed the libXau package before upgrading Xorg to 6.9.0, but I had also previously copied the libXau modules from their install location to /usr/X11R6/lib/. When I upgraded to 6.9.0, I got some error messages referencing the libXau modules, but I ignored them. That was a bad idea, I guess. startx still failed, apparently before it got to the point of opening the log file.
Just now, I deleted the libXau modules that I had copied to /usr/X11R6/lib, then I removed the Xorg 6.9.0 packages to make sure everything was clean. Then I installed Xorg 6.9.0 again and tried to startx. This time, no errors, and KDE 3.5.2 came right on up. I'm using it now.
This has been a good learning experience. Thank you for your help.