Graphics Ceiling?

I recently made a mild upgrade to my PC, by upgrading my old nVIDIA GeForce MX4000 to a 6200. Yeah, that's not much at all, but there's a marked improvement in gameplay, I can tell you.

What I was wondering, was how much improvement would I have experienced had I been made of money and got a 7900? Is there a specific GPU ceiling for improvement? Like, for example, you wouldn't see any noticeable difference with a 6800 or more, etc.

I know Chapter 2 will have a higher minimum sys req, and take advantage of the better GPUs, but I wonder how good ERT can be.