XFCE is designed from the get go to be very light weight. On my laptop I average ~300 MB Ram usage idling with XFCE. MATE is the spiritual successor to Gnome 2

XFCE is designed from the get go to be very light weight. On my laptop I average ~300 MB Ram usage idling with XFCE. MATE is the spiritual successor to Gnome 2, which isn't as light weight (but they haven't tried to add things to make it bloated, and gnome 2 was never too resource intensive). It is taking ~700 MB idling on that same laptop.