look for grub.conf in /etc or it might be /etc/grub/grub.conf. Redhat puts it all over the place
You need to add vga=ask to the end of the kernel line.
When you next boot it will give you diffrent options select one and right down its number then add that number to the vga=. Don't forget to convert in to decimal. (a=10)
kernel /vmlinuz-2.4.21-sorsha1 root=/dev/hda6 vga=10 /dev/hdd=ide-scsi