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I think that one key point might be if you're enabling 3d acceleration or not. I use XFCE and without 3d acceleration, it uses about 30MB of RAM on a clean install/boot. But if I enable 3d acceleration (on an intel integrated graphic card) it will use an extra... 60MB of RAM ! (with xorg 6.8.2 it was just 25MB extra).
To disable/enable 3d acceleration you can comment/comment out the line in /etc/X11/xorg.conf where it says:
(if you don't have that line in the "Module" section you should also add at the end of the file:
To check if 3d acceleration is working, you can type at the command line "glxinfo" and in the first few lines you should see "Direct rendering: yes" if it's enabled.
Disabling DRI is a good way to save RAM in older computers. It's not much used in Linux anyway (except google earth, some games,...)