i assume hdinstall here. if that is not the case ask further.
for 1024x768 and 256 collors in console you want to edit /etc/lilo.conf so the line vga=normal is vga=773 .
in X you edit the /etc/X11/XF86Config (or run xf86cfg) and change the default to 1024... y putting a # infront of all lines that have unwanted values.
check s that all the lines belonging to each setting is comented out. otherwise Xwill complain/crash