1440x900 resolution in settings

[Mihamina Rakotomandimby (R12y)]

Hi,
I have a gentoo installed xfce 4.2 and when I try to set the reolution
via the xfce settings, there is only 1152x768 as highest on the list of
available resolutions.
When using other X stuff (fvwm & gnome) the resolution is set properly
to 1440x900.
X settings (xorg.conf) is OK, 1440x900 is listed and as I said, it's
working on others.
What would I have to do to have 1440x900 listed as choice in the XFCE
setting?
Thank you.