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You probably want xfce as Maciej suggested. A couple other ones to try, depending on whether you're comfortable stepping away from full-featured desktop environments:

• Fluxbox - it looks sparse, but it's really efficient, and the tabbed windows feature is quite nice.
• Enlightenment - might not be much lighter than gnome in memory, but it reputedly light on CPU.

Both have long-ish histories and are very good at preserving screen real-estate,