I have ATI Radeon 9550 card, with the open drivers.
In the display properties my screen is not detected,shows UNKNOWN,and don't have 1152x864 which is best for me.
How can i enable it?
You might want to install the AMD Catalyst™ 11.11 Proprietary Linux x86 Display Driver proprietary drivers. Latest release is rather stable. You can get them at AMD's site. It should allow you to set the resolution you want.