Display problem(can't start x)

I fail to start ubuntu 10.04 with X after the recent os update was made.

When I boot the system, the following message comes:

"Ubuntu is running in low-graphics mode : Your screen, graphics, and input devices settings could not be detected correctly. You will need to configure these yourself."

Then if I click "OK", I see some options to configure settings, but nothing of them works. In the end, I am moved to the command line login.

I have tried to find a solution from net search and I have made some tries but failed. It seems to me that video card and its driver are correctly detected in spite of the above message. I don't know what the problem is.

When I execute 'xrandr', I have a message like "Can't open display".

Here is my hardware info:
Monitor : LG L204WT
Graphics card : ATI Radeon HD 4200

Thank you.

Seungtaik