OK. I know DX12 is going to stack VRAM, however, older games, Witcher 3 and GTA 5 will not stack, they could but the developers would have to rewrite the entire engine. So that's never going to happen. Future DX12 games will be able to take advantage of stacking, again though, this is solely dependant on the developers and if they implement that within the game. On a different note, you'll only get 100% of all the benefits of DX12 with the new nvidia 900 series cards.If anyone tells you anything different to what I've said here then correct them, if they don't listen then forget them, they are full of bull.
So to anyone out there who says don't upgrade yet, well, they obviously don't want to take full advantage of DX12, which is pretty dumb in my opinion.