If you use lilo, you can do it one of two ways. To only do it temporarily, wait until lilo is running. Then if you change the selection to something else, then back to whatever you called your linux boot, so that you get that thing's name in the prompt (mine is called LFS, yours could be linux or mandrake or whatever), you can just type " mem=nopentium" (w/o the quotes) and hit return. Then, once Linux is up, check /proc/cmdline to make sure the option got passed.
Otherwise, you can edit /etc/lilo.conf, and add mem=nopentium to the append= line in the proper section. After you've saved the config file, run /sbin/lilo -v to make the changes active.