I don't think you can use the "Virtual" line without a corresponding "Modes" line. For the login, X will default to the first resolution defined in the “mode” entry. Therefore, you must select the resolution you want (i.e., “1280×1024″) and move it at the first position. The “Virtual” entry is typically used to have a larger desktop resolution than screen resolution (you can reach the zones “outside the screen” by moving your mouser pointer to the edges). Your Virtual section should have the same size you want for the login resolution (say 1280 1024).
Another way to control GDM's resolution is to use xrandr
You can direct xrandr to set a different resolution using the following command
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xrandr --output VGA --mode 1024×768 --rate 75
Add undetected resolutions
Due to buggy hardware or drivers, your monitor’s correct resolutions may not always be detected
If the mode already exists, but just isn’t associated for the particular output using the following command
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xrandr --addmode VGA 1024×768
Note: Changes you make using xrandr only last through the current session.
Set xrandr changes persistently
There are several ways to make xrandr customizations permanent from session to session:
A user’s ~/.xprofile file is executed on Xorg startup if it exists and is executable. You can copy and paste xrandr command line strings into this file so they’re executed when you log in
Both KDM and GDM have startup scripts that are executed when X is initiated. For GDM, these are in /etc/gdm/, while for KDM this is done at /etc/kde4/kdm/Xsetup. In either case, you can paste in an xrandr command line string into one of these scripts.
This process requires root access and mucking around in system config files, but will take effect earlier in the startup process than using .xprofile, and will apply to all users including the login screen.