While Linux was once the operating system of command line enthusiasts only, there are now several popular graphical desktop environments, including Gnome, XFCE, and KDE. Of the three, XFCE is the lightest-weight, meaning it's designed to run well on older PCs with slower processors and less powerful graphics cards. Gnome and KDE include more desktop effects, some of which might just be pretty to look at, while others can actually make improve productivity.
The KDE developers team is preparing to release the next generation of KDE. While KDE 3 is often described as an interface that Windows users will find familiar (the taskbar is on the bottom of the screen, there's a Windows-like start menu, etc), KDE 4 looks more like a cross between Windows Vista and OS X Leopard. That's because it comes with some amazing visual effects built right in like window transparencies, and a task switcher that looks a lot like Exposé on OS X.
KDE 4 also includes a new icon set, a new file manager, and a bunch of under the hood changes that most users won't notice. And it accomplishes all that while using less memory than KDE 3. That means you can run KDE 4 well on older machines, or newer computers like the Asus Eee PC which come with slower processors and a relatively small amount of RAM.
The video above shows Trolltech employee Jesper Thomschütz running Kubuntu with KDE 4 release candidate 2 on an Eee PC. You can download that LiveCD disc image yourself and test out KDE 4 on your own machine. KDE 4 is officially scheduled for release on January 11th. It will probably be a little while before you see Linux distributions like Kubuntu or OpenSuse coming with KDE 4 as the default graphical user interface, but you should be able to install stable KDE 4 packages to your existing Linux system starting next month.