I could be wrong, but I think the boot screen is loaded by the line 'message=/boot/message' in the /etc/lilo.conf file. If you look at /boot/message you should see that it's a symbolic link to /boot/message-graphic. There is also a file called /boot/message-text and I'm guessing that if you change the symbolic link to point to the text file you would get that instead of the boot splash. You could also change the /etc/lilo.conf file to read 'message=/boot/message-text'. If you modify the lilo.conf file, make sure you run the lilo command before you try rebooting.
You can also change the boot configuration using DrakBoot in the Mandrake Control Center - under the 'Boot' tab.