I succeeded in installing Gentoo and XCFE on a now 16 year old SGI Indy (one 150MHz MIPS R5K, 160MB 60ns ram, Fast SCSI HDD, framebuffer-only Linux graphics driver). It's not snappy by any means, and it makes it very obvious which programs "do it right" and which ones rely on hardware to cover up bad algorithms. Yet none the less, it boots right up, I can login and start the GUI no problem, I can play music back and do ssh consoles, IRC (with a gui client no less) and even browse the web with a WebKit browser.
Can't imagine what even kde 3.5 would do to the poor thing, let alone kde 4 or gnome 3. Kde 3 might actually have been borderline usable... When my desktop had 256M ram, kde 3 started grinding the hard drive after about 6-7 tabs and 3-4 konsoles were opened.