Well I had working KDE 1 on a machine with 16MiB, and you can setup a working X11 + FVWM with 4MiB of RAM. Obviously using older versions of Linux kernel and userland.
More ridiculous could be a full GUI OS on a machine witch less RAM, like an Amiga 1000/500 or a early Macs.
I'm talking about the current versions, not old stuff, i also had KDE 1 working on a 32MB machine.
I do not expect current versions to be easy to run even in 32MB (although i've heard - but didn't verify - that Xorg can be configured and built from source to use as little as 600KB) but i'm curious where those hundreds of MBs go even for something like Xorg + Window Maker.
More ridiculous could be a full GUI OS on a machine witch less RAM, like an Amiga 1000/500 or a early Macs.