This might help:
I don't know that you will easily find benchmarks comparing Gnome, KDE, LXDE, and XFCE on the same platform. My cursory search didn't turn up anything.
You might try running GkrellM
under the desktops you're interested in and comparing the outputs. If your distro does not include GkrellM, it's likely in the repos.
Just based on my personal experience, I think that KDE, especially KDE v. 4 with Akonadi, is heavier than Gnome.
XFCE is lighter than either of those and LXDE is almost as light as a window manager, but I can't cite stats.
I am not a fan of Akonadi. I think it is a lot of overhead that accomplishes little, but, then, I'm a Fluxbox
kind of guy.
Except for the Akonadi thing, I tend to prefer KDE over Gnome; I find KDE's tools (like the system settings, infocenter, etc.) far more useful than the Gnome equivalents.