XFCE is a light graphical interface that can be use when your software does not support KDE or GNOME or just if you like it more that the others.

Actually XCFE is a nice and complete interface, I have not used it too much as I use GNOME the most of the time.

Surfing the web I have found these two articles, with them you will improve the look of your XFCE desktops, and also improve some of its performance, one is focused on Debian Etch and the other on XUBUNTU (Which is Ubuntu with XFCE) this version of Ubuntu is oriented to support old hardware computers. Check these two articles.

For Debian

For Ubuntu