First of all, you'll want to pass the kernel parameter "dimensions=1280x800" (the "video=" parameter seems to do the same thing?)
Edit: a source says you can pass this to the kernel instead:
vga=771
You need this in your xorg.conf:
ModeLine "1280x800" 80.58 1280 1344 1480 1680 800 801 804 827
and this program:
http://perso.wanadoo.fr/apoirier/
execute thus:
855resolution 5c 1280 800
Google could have done this for you
Samsara