I Have a ATI Radeon X600 Card (RV348) and Samsung SyncMaster 550v
I Install Ubuntu 12.04 with a Plasma Monitor I can´t see the desktop
I can't see the GRUB options; I would like to change the Video configuration but xorg.conf don't exist
I can't tell if you have one monitor working or not from the question. Assuming one monitor is working, I had a similar issue with Ubuntu 12.04.
When I go to "Displays..." my second monitor isn't detected so I could not enable it. However, when I searched for Nvidia in my application directory (Dash Home) I found the "Nvidia X Server Settings" utility. That utility detects my second monitor, and once I enable it there it works just fine.
So, assuming you have one monitor working and have already installed the correct video drivers, search Dash Home to see if there is a config utility.