too bad i havent seen this thread before..
Well anyways, ive not tried debian or knoppix, but i can only say, give gentoo a try, even as a new linux user...
Ive started with a suse install back in 99, used it for a week and wiped it again i guess.. didnt had a good pc/not enough hd space for dual boot etc ack then...
since 2001 im using linux again on a 2nd comp, i tried mandrake cause it was meant to be easy and good..
well, i could get it to work.. since 1 year im using gentoo though, and all i can say is.. skip mandrake and the like, gentoo is such a nice system, you can inform yourself about it at www.gentoo.org
it has no graphical installer, thats right, but it has great documents and a great community, so installing without a gui wasnt a real problem.. it has disadvantages, eg for gentoo you need quite much time to get some things to work, it doesnt do many setup related things automatically.. but this has advantages, everything will be as you like it and you will learn alot more about your system and linux than you do with mandrake/suse/redhat etc - also it has a great package management system called portage..
however your pc should have a good internet connection and a fairly fast cpu, as you need to compile most or everything you want to install etc..
cephas can probably at least partly confirm this