The ATI fglrx driver includes a command line tool that will allow you to modify the xorg.conf file. This will allow you to enable and disable the composite/svideo out vs the VGA out for the card. You might wish to google for "fglrx composite out" or "fglrx tv output". That's what I had to do. Just being told how to get into an editor for the xorg.conf file will not be enough. For doing this kind of stuff, I will normally open up a terminal window and do a "sudo su -" in it, to have root access.
Just be warned, the current LinuxMCE has a bias towards NVidia based cards, because of 3D composting capabilities that are not included in the ATI driver. That is what allows the fancy UI2 menus that are shown off in the video you saw. I eventually gave up messing with the ATI video driver when I found out that the ATI based system board I was using couldn't run both the VGA and Composite/SVideo at the same time! Good thing I didn't buy that $300 case that has the VGA LCD on the front! I am presently using the ATI based machine as my core in the "server room"(basement).
And, yes the video shows you what all this work to set it up is for :-) It's unfortunate that the installation is not as simple as it could be, but things are actually getting better.
The actual commands I would do in the terminal window are like this...
sudo su -
Then I would start up an something that needs to display to the screen like xterm just to verify that root can display to the screen. That's what the xhost line is for. If xhost +localhost doesn't work, then do xhost + instead. The reason why I don't recommend that, is that it allows anybody on any machine to access your X server. The +localhost only allows access from your local machine. 127.0.0.1 should be equivalent to localhost. You can close the extra xterm right away if you want, it really only needs to be done once to verify that the xhost line is correct, after you determine the proper xhost command you don't need to perform that test again.
Hope this helps a bit..