Knoppix is not very suitable for hard disk install, and if you are dealing with limited RAM then I'll say that it would be even crazier to try to install Knoppix that it usually is, since you will not be getting any of the benefit of all of the Knoppix apps, just the bloat. While there are many different Linux distros, if you like the basic flavor of Knopix, consider installing Debian, the distro that Knoppix is based on. I suggest that you try an install of the "testing" version of Debian, which is currently also known as "Lenny". As long as you have an Internet connection available to install from, I suggest doing a "net install" of it with this ISO file. See Debian.org for more details. If during the install process you don't want a GUI, don't take the Desktop option. If you want to try Gnome (smaller than KDE) then take the Desktop option during the install. Of course, you can always install other GUIs later with apt-get.

Debian is far from the only good choice, but as this is a Knoppix forum it seems a good place to start.