I'm trying to understand how video drivers are integrated into xorg.

In a smoothly-running Ubuntu install, I find the video drivers in /usr/X11R6/lib/modules/drivers. They all have the extension ".o". Among the many there, I found vesa_drv.o, radeon-drv.o, and ati_drv.o . The xorg.conf file includes <driver="ati"> and the screen behaviour seems 100% correct.

In my nascent install of Arch, there is only one driver in the drivers folder---this time: /usr/lib/xorg/module/drivers --- it is vesa_drv.so . Teh xorg.conf has <driver="vesa"> .
with this setup, the system crashes whenever I attempt to change screen resolution (using XFCE4)

In searching here and elsewhere, I cannot understand why the Arch version of xorg would have only one video driver. Where does one get all the others?

Finally, what is the significance of .o vs .so ?

Judging from the /usr/X11R6 prefix, the Ubuntu version is the monolithic 6.8 tree. Arch uses the 7.0 tree which is modular and in my opinion, too modular. Every single driver for every different thing you could imagine is a seperate package, not the best solution but that's how it goes.

Anyway, you're looking for the xf86-video-radeon package. pacman -Ss xf86 | less should give you a list of the drivers, go from there

That did it!! Thanks very much for the help.
I'm really liking Arch, but that's another story....