Changing graphics card causes applications' and window manager's fonts to change?

[Adam Funk <a24061@yahoo.com>]

Yesterday I shut my Debian box off, swapped in a different graphics
card, rebooted, went into a console and changed one line of
XF86Config-4 (the Driver line for the card) and did "/etc/init.d/gdm
restart".  The screen resolution is the same and the monitor is the
same.

The GDM login screen came up in a slightly larger font and the fonts
used by XFCE4, Firefox and the KDE apps that I run all came up
differently.

Why does just changing the graphics card (and driver) while keeping
everything else constant have this effect?

Thanks,
Adam