What is really small and low usage? I have GNOME and have also used KDE.
Neither of those are good choices for "low useage," in my experience. KDE is probably the worst choice. Fluxbox and XFCE work pretty well on an older computer. I'm using XFCE on a 450 MHz CPU / 128 MB RAM laptop and it runs pretty well. Fluxbox runs better, but I like XFCE's methodolgy/menus more.
My favourite is WindowMaker, but xfce and FluxBox are good too. xfce is pretty, and FluxBox wins polls for being the best.
And for the record, those aren't "desktop environments", they're "window managers". I think that KDE and Gnome are the only desktop environments.
I can't say anything positive about XFCE, but Fluxbox is pretty nice.
Trying to change the user menus on xfce was a pain, so I gave up and moved to WM, which I like. I still haven't tried Fluxbox, but I have run KDE and WM at the same time (for whatever reason..)
I use fluxbox for my server's VNC desktops. It works really well. It's the most lightweight window manager I've used.
Here's a comparison between several window managers:
http://www.windowmaker.info/features.php?show=performance
Fluxbox and Blackbox should be roughly the same.
If you want to have some fun, install Ion. I think I could really love Ion if I got used to it, but it's difficult to learn.
I like OpenBox3. :/
I use XFCE and I love it. Runs fine on my 400MHz, 384mb RAM, 8mb video-card computer. The menus suck balls to change, like rabbit said. I still don't know how to do it. ><
RTFM.
I did, but the FM doesn't really apply to anyone but root. I RTFM and I still couldn't change the menus on my user account.
You can use most window managers with Gnome, it just comes with Metacity by default :)