Change Resolution in Fedora 10 to undetected value

I'recently bought a Thinkpad T500. It came with Vista but I've immediately installed the last version of Fedora.
After the installation, that didn't present any particular problem, I wanted to switch the resolution to 1280x800.
You can set the resolution directly in Gnome, under:
System -> Preferences -> Hardware -> Screen Resolution.
Unfortunately my desired resolution wasn't automatically detected, therefore I should to set it manually.

To do this follows this steps:

$ gtf 1280 800 60

# 1280x800 @ 60.00 Hz (GTF) hsync: 49.68 kHz; pclk: 83.46 MHz
Modeline "1280x800_60.00"  83.46  1280 1344 1480 1680  800 801 804 828  -HSync +Vsync

$ xrandr --newmode "1280x800_60.00"  83.46  1280 1344 1480 1680  800 801 804 828  -HSync +Vsync
$ xrandr --addmode LVDS "1280x800_60.00"
$ xrandr --output LVDS --mode "1280x800_60.00"

Where LVDS is your video output.
To get all outputs of you sistem use:

xrandr --prop

That's it! To persist this setting, add it to the beginning of /etc/gdm/Init/Default.

Don't forget to correctly set also your dpi, going to
System -> Preferences -> Look ans Style -> Look
and then in the tab "Fonts type" click on the button "Details"

To compute the optimal dpi for you system, use this formula:

dpi = sqrt(w^2 + h^2) / d

where:
* w is the screen width resolution in pixels,
* h is the screen height resolution in pixels and
* d is diagonal size in inches.

In my case the value for a 1280x800 resolution on a 15.4 LCD Screen was 98.