Windows pops up a memory warning when it reaches 6-7GB if page file is too small.
I've seen memory usage @ 92% (on 8GB RAM) with GTA V, Skype and one Explorer tab open. So it does use quite a bit of resources overall, but 8GB should still cover it. And increasing the page file size should get rid of that annoying warning.
Glad my GTX 680 was a 4GB model, as it's using up to 2.5GB video memory @ 1080p with the settings I've applied. Not a modern card anymore but runs the game great at that resolution.
Have seen over 3.5GB on occasion using my GTX780 6Gb and Afterburner @2160*1080.
Shadows of Mordor on ultra textures? 5.4GB Video memory!