Easy. I use Openbox as window manager. And lxpanel as panel. So yeah the panel is from a DE already.
Then.. I use K3b for CD burning, Konqueror as a light browser, Dolphin as file manager, Ktorrent as torrent client, Ark as archiver, Gwenview as picture viewer, Kwrite as text editor. Thats for KDE (4.2.4).
Then I use pidgin for messaging (that can be gnome,xfce,whatever, gtk), claws-mail for email (gtk), monodevelop as IDE (gnome), Gedit sometimes for text editing (Gnome), Xchat (gtk), Gimp (GTK)..
You know.. I don't really care about such stuff. I always try to use KDE as base , and some GTK stuff as support apps, and if I REALLY need..then I install a gnome app with a hell of dependencies (yeah I hate gnome, so i cant really help it.. I struggled with it for months when KDE 4.0.0 came out).
So what I want to say? Even with this much "cross-app", I only get ~200mb or ~300/400mb memory usage. What is... OK I think. Even if I got 2 "workspace" full... :)