Well, I tried to use the fglrx driver, but in system preference, restricted driver it says : your hardware does not need any restricted driver so I can't use auto install.
I also tried to install it myself (from ati website version 8.4) but I have a black screen at boot
I had the same problem at first with my X1250 motherboard graphics.
It seemed it has troube ldetecting the monitor display modes over a long cable (EDID issues I think), a short cable and it was fine.
Also, the updated restricted driver is now fine.