Cinnamon doesn't eat a lot of system specs, at least on my computer (and my computer usually does odd things). I'm currently running KVIrc, Firefox + Flash and a Terminal with top, and it's just using ~700MiB of RAM, and ~14% of CPU... I never tried MATE on my "real" computer, just on Virtual Boxes, but perhaps I should give it a try.
I also love XFCE, my other OS is Xubuntu which I customized a bit and it's also very fast... And KDE is the most heavy of them, AFAIK.
Catbuntu






