Ubuntu – the difference between fglrx and fglrx-updates

In theory it's possible: first, you should modify /etc/X11/xorg.conf, I think you need to modify things in the "Device" section (well, Xorg nowdays works fine without xorg.conf, as you noted too, so the other solution is rename xorg.conf so X won't use it).

But it's only the half part of the story, the other part is the kernel driver. I would "blacklist" fglrx as a kernel module, so it won't be load, when you don't need it: /etc/modprobe.d/blacklist.conf

I don't have a setup for ATI card in this way, but I have similar for nvidia, to switch between nvidia's binary driver and nouveau (open source driver for nvidia cards).

The 'post release updates' will install Catalyst 11.9 which is the latest version while the 'ATI/AMD proprietary FGLRX graphics driver' will install Catalyst 11.8 which was the driver at the freeze stage of 11.10 development

There has been issues with the 11.9 driver with Unity (see ubuntu forums here) so I suggest you install the 'ATI/AMD proprietary FGLRX graphics driver'.