I found this tidbit somewhere. It might be what you need.
Post-install tweaks :
DISPLAY :
We want to use the real native resolution 1440x900 at 60 Hz (don't try to "overclock" the 60 Hz refresh rate !!).
We get the right Modelines with gtf software, like this, in a console :
# gtf 1440 900 60
and it send us the correct modelines as output :
Modeline "1440x900_60.00" 106.47 1440 1520 1672 1904 900 901 904 932 -HSync +Vsync
Let's copy this line into "Monitor Section", in /etc/X11/xorg.conf (or XF86Config if you still use XFREE 4.3...), and add Mode "1440x900_60.00" in Screen/Display Section (See my xorg.cong file below).
# init 3 and # init 5 to restart X server, and you have the right resolution. You can check this with xvidtune soft, which will give you the resolution, and the refresh rate...
# xvidtune
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