[lixe]

Hello,
I just followed this guide http://wiki.xbmc.org/?title=HOW-TO:_Inst...ep-by-step , but i can't find anything about the xorg.conf configuration fot ati...

[althekiller]

You're going to get a better response, quicker, from the ubuntu (or w/e distro you chose) forums in regards to system level stuff, than you are here.

[b01]

Like althekiller said, seek help in the Ubuntu help forums (https://help.ubuntu.com/).
But I suggest you take a look at this website to configure your Ati card in Ubuntu Unofficial Wiki for the ATI Linux Driver. I use this instead of ATi or Ubuntu's site, as it is up-to-date.