I'd say go for gentoo.
This will make you cry several times, and you will pull your hair out, but as you said, you want to
learn linux.
Something i feel is not easily acomplishable with mandrake, redhat or any of the other disro's that has everything neatly packed for you and dumps you straight into x (GUI) so you'll never actually get to learn how to use it.
You computer may be a bit slow, as gentoo is a source based distro, so it compiles each and every single line of code you're going to be using.
The community's great though, you'll be able to get lots and lots of help from there.
www.gentoo.org