First of all, I'm no computer expert, so I apologize if my question seems too trivial or somewhat ill-posed.
After googling a bit, I've found that the GeForce GTX 675M Discrete Graphics (DDR5 4G VRAM) is a better video card than the Radeon HD 6970M (2GB GDDR5). This seems pretty obvious.
However, all references I've looked compare them without taking into account this specific amount of memory (4G for GeForce, 2G for Radeon). They just compare "GTX 675M" and "Radeon 6970M". I don't know if it's a common assumption to use 1G for both or what (couldn't find it either).
My question is: How big is the difference between those cards, taking into account the memory? I'm specially interested in the FPS Rate for demanding games, so you could use that as a measure.