LXDE is a fairly new desktop for Ubuntu and other flavors of Linux that has been around since 2006. It uses less memory than Gnome or KDE. According to some reviewers, LXDE even trounces Xubuntu in memory tests. I have and old laptop and I gave LXDE a spin, and yes it is lighting quick. It sports a usable, and intuitive interface as well. I can foresee something like LXDE running on a netbook or some other lightweight computer.
For more information about LXDE, here is a nice review: A Review of the LXDE Desktop for Linux
To install LXDE on an Intrepid Ibex Ubuntu or newer run:
sudo aptitude install lxde
I run this Ubuntu 9.10 with no problems.