I am willing to bet that a gfx card with 3GB GDDR could handle Fallout 4 with HD texture packs. If not a 4 GB card would be able to. The thing is, if you go insane with the mods, you could bring a Titan X (12 GB card) to its knees. Any mod with a graphics component is likely to eat into your VRAM. Once the card hits its limit and it starts swapping video assets to regular ram or God forbid, disc, the frame rate is going to start being measured in Seconds Per Frame instead of Frames Per Second.