This stage is optional, depending on whether your system will cope with the Gnome services and applications that Xubuntu includes.
In any case, you should consider turning off the roaming feature in System Tools -> Network before applying this command, since it will save a little Terminal editing later. Just make sure that your ethernet interface is enabled, and set to DHCP (or static if that is what you use).
These Gnome dependencies and heavyweight System Tools (and Games) can be removed with the Pure KDE Remove Xubuntu command (minus the end part relating to reinstalling kubuntu-desktop).
Remember, this command removes Firefox, so you will have no more internet access after this command. If this is a problem for you, append the command with && sudo apt-get install firefox. There are other browser options too: see Web Browser.
If you want to maintain some of the potentially useful Gnome stuff, don't follow this "Pure KDE" advice. This removes gksu, Synaptic, Update Manager and some of the other GUI controls for Network Settings etc. Nevertheless, I think it's better to get rid of them to keep things slim. Components can be added back in customisation at a later stage.