emacs and vim are so different from "desktop apps" that you won't notice the difference.

For everything else, most people use the KDE version of whatever app they are looking for. I haven't used any applications other than emacs, urxvt, firefox, and amarok for years, so I can't really tell you what those are.

Edit: actually, I can tell you some things. There are themes that make Firefox look like a KDE app, and of course there is Konqueror (I tried 4.0 for a while, it was much nicer than FF2.) There is also a GIMP equivalent for KDE, but the name escapes me. My advice is to install a KDE 4.1 version of Kubuntu, and click stuff in the menus.