8GB sounds like a lot now, but every bit of ram helps to run a game smoothly when it all has to come from a cheap 5400 rpm hdd.
When you check how much memory a game uses in windows it doesn't include the video ram, and probably also not the disk cache that windows is using to help your game run smoothly. My laptop has 3.1GB in use while just browsing vgchartz.
Sure you can optimize a lot, use repetitive textures everywhere, identikit building pieces, long loading times going in and out buildings, when popping up menus etc, aka Skyrim on consoles. I hope this gen will bring more variety to the worlds, procedural generation for geometry, trees, clouds etc. You need a lot of memory for that.