oops, I read your post too fast. Webmin can show you that stuff, buuuuut...... only with it's default theme, which I hate. Disk space would be under the Hardware tab in Webmin, but ram, cpu and some other things are one the front page of the default theme and I can't figure out where to find that stuff in the theme I like, but oh well. Also, webmin does not (that I know of) keep a history graph of the values.
also, if you end up writing a php page to do this (and maybe make a daemon or cron script that monitors and stores a history in a mysql or sqlite database) you might consider registering it on sourceforge.