For a old machine with not much RAM, your best bet is to skip Knoppix (even though is too cool) and go straight to Debian. Here is the CD ISOs for installing Debian Sarge http://cdimage.debian.org/pub/cdimag...i386/20050117/. If you haven't seen the new Debian installer or have heard in passing that Debian was difficult to install, well thats not true anymore. Download the big image if you have the bandwidth and then burn it & boot it up. Here is a screenshot gallery of the install so that you will have an idea of what to expect http://shots.osdir.com/slideshows/sl...se=184&slide=1.
I hope this helps.
James