It just depends on what you wanna learn.
A full-blown Ubuntu won't be the best i think. Rather go for Xubuntu which is alot lighter. It's really easy and possibly one of the best bases to start with linux.
But if you wanna get performance from the box i would rather try and set up something else. And not everything is that hard. Here's a few examples of the famous ones:
Debian is what Ubuntu is based on. It's pretty nice and simple and it has APT (package manager) which is super-nice. Pretty easy and effective. You have more control of what's installed that with the Ubuntu distros, and combined with XFCE, then it's pretty nice. Debian also has nice and understandable documentation.
Slackware has a nice installer where you can just choose all the packages you want in the base system. But you might wanna have google open. And you'll learn what Linux is all about

(it's the oldest surviving distro
You can go for a old-school gentoo install and build the whole damn thing from scratch, like bootstropping, chrooting and compiling kernels (and i promise you that you'll learn A LOT!). Gentoo is also really nice for setting up a custom distro. That does exactly what you want it to.
Old post, but whatever, i was a bit bored