Generally, I prefer KDE apps to Gnome apps, though I do like Totem (the Gnome media player). I have both Gnome and KDE installed on my computers for the libraries, but I use Fluxbox
as a window manager.
For something lighter weight than either Gnome or KDE, but that is still a desktop environment rather than a simple window manager, consider XFCE
. It comes with Slackware and is in the software repositories for most major Linux distributions.
I think that KDE 4 will have trouble on that box as an interface--it's more resource intensive than I like--but you can run KDE apps under Gnome and vice versa (or under Fluxbox or any other window manager) as long as the libraries are in place. (Think of "libraries" as functionally sort of like Windows *.dll files--they are shared by multiple applications.)
For example, two of my computers came with Gnome on top of Ubuntu. I downloaded KDE to have the KDE libraries, but almost never use either the KDE or the Gnome interfaces.