ATI support
Where were you looking for documentation?
Support for ATI does seem to be a bit lacking - though I understand that is because ATI haven't written workable drivers. Nvidia on the other hand have embraced the Linux market and installing Ubuntu on my 20" widescreen machine posed no issues at all. I'm not a hard-core Linux junkie but I managed with very liitle effort on my part.
As an aside - last time I rebuilt my games machine (XP), it took me ages to get off 640x480 on my 26" TFT television as XP set it as default and I had to fight tooth an nail to get wireless networking up and running. To me OS issues are just swings and roundabouts - XP has it's advantages ove Mac/Linux and vice-versa.