i guess u wuld have got all the info needed here
but could u just specify the board and the onboard VGA u r usin?
in most common cases
# XF86config will do the trick
use the vesa mode if u have a real old board and VGA
and just try to lower the max resolution available to something like 800 x 600 instead of the 1024 x 768 which is set as default
and if there is an option in the memory to increase the shared video memory size, do it so to get atleast 8MB for video( provided u r usin an onboard video)