It depends on what you want (diffent folks, different strokes, so to speak)
If you are a curious home user looking to dable in the world of linux while still having a working system, ubuntu is the way to go. It is a strong distro with a great, helpful community.
Check out www.ubuntuguide.org for tips on how to get up and running.
The guide is for ubuntu 5.04 and the latest version is 5.10; you can either run 5.04 (which will be supported with updates for at least another year) or you can be daring and try use the page as a guide to point you in the right direction on how to do things in 5.10. I would recommend using 5.04, though.
If you want a more "geekish" experience, debian sid is nice.
Knoppix is just a live CD version of Debian. If you install it, it becomes Debian, it think, or at least it is so similar you might as well just install debian.
Good Luck!
I have a Dell Precision 340 which i have windows XP SP2 on. I also however need a recommendation on which linux to put on a second partition.
My main reason for requiring linux is to get familiar with it as well as to program C.
So far im considering Knoppix, Ubuntu or Debian.

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