I'm thinking of buying the iMac 2012 base 21.5 inch, http://www.notebookcheck.net/Mobile-Graphics-Cards-Benchmark-List.844.0.html this list clearly says the nvidia GT640M (nr. 80 in the list) is superior to the Radeon HD 6570M I'm using now (nr. 129 in the list). But what does this mean in real world performance? Is it a huge boost? will I even notice it? Is it a weak card (512MB RAM)? How does it compare to the Radeon 6570M? Thank you.