Thx for yur mail,
I post today the log you ask...
it's quite amasing : if I use "nv" I have my screen but no glc because glxgears says I have no glx used.
and if its "nvidia" it crashes and it seems that it doesn't find the screen.
My OS is Mandrake 9.1, the version of Xfree seems to be the 4.33
My workstation is a dell precision 450 with a xeon 2.8 Ghz and an nvidia quadro4 900 XGL
thx again for your help.