The Vista drive died on my desktop so I decided to install Ubuntu Jaunty x64 instead. The computer has a pair of HD 4670 in CrossFire. I had to go through safe video to install it, otherwise the screen just blanks on me.
X seems only recognising resolution up to 1280x1024. It's not detecting/using the 1920x1200 on the KDS 24" monitor. I fudged with xrandr and xorg.conf but couldn't get anything to work better. Then I moved on to try fglrx (open source ati driver doesn't support 4670 yet). The one in the repository made X fail, since fglrx.ko isn't at the place it's supposed to be. Don't know if it's a x64 issue or not.
Then I downloaded installer from ATI, not hoping for much, but it works like a miracle! Installer went through fine. Log off, log on, voila, I got 1920x1200! I never really was fond of ATI's drivers, but this one did shine.
Tried a H.264 1080p video under Movie Player (Totem) and it ran fine. Action sequences were smooth, but it would slow down visibly if the control menu is shown. Probably a player bug. In fact if the controls are gone and I move the mouse to get them, the screen would go back to desktop background briefly before returning to video with control bar. Annoying.