So, for the first two, which one has better graphics? and is it worth it to spend $50 for another gb of video memory?
540M and 6650M trade blows, but overall 540M would be a little bit (very little, very hard to see/spot the difference) faster because of the benchmark scores. In games they will trade blows. It is not worth it to get 2GB VRAM for $50, because 1GB is all needed for Laptop graphics. That even includes the 580M or 6990M, except you are playing and having screens like QHD, WQXGA or running eyefinity (which you guys rarely do for laptops, only desktops).