Mmmmm.... Some of that is probably disk cache as well, but in any case it's not leaving you enough to run the game. I have seen youtube videos of Amnesia running in on X3100 systems with Gnome3 (in a 640x480 window) but I have to say, it looks like your system is just too far under powered for this game
. If these youtube guys are using similar spec machines to you, I'd say they were lucky. You could try and ask them what they did to get it running.
Having said that, KDE & Gnome/Unity are quite heavy. You could try a lighter Desktop Environment like LxDE or Xfce to free up some resources. I believe there are spins of Ubuntu that come with those, and OpenSuse gives you a choice of KDE, Gnome or Xfce when installing.