Bug#481063: linux-image-2.6.25-1-686: vga=791 gets 'undefined video mode number'

["Jeff Spicoli" <jspicoli@gmail.com>]

I had the same problem, here is what worked for me:

apt-get install hwinfo

hwinfo --framebuffer

Look at the Mode lines, find the one I want:

Mode 0x0323: 1024x768 (+4096), 24 bits

convert 0323 from base 16 to base 10

put result into menu.lst kernel line:

vga=803

You don't need hwinfo to get the modes, you can set vga=ask and then convert the numbers you see to base 10.

HTH...