comparing

GPU Engine Specs                 640m           6  635m             2
CUDA CORES                       384            /  up to 144
Graphic clock                    up to 625MHz      up to 675 MHz    /
Texture Fill Rate (billion/sec)  up to 20.0     /  Up to 16.2
Memory Interface                 DDR3\GDDR5     /  DDR3
Memory Interface Width           128 bit           up to 192 bit    /
Memory Bandwidth (GB/sec)        up to 64.0     /  up to 43.2
OpenGL                           4.1               4.1
Bus Support                     -PCI Express 2.0   PCI Express 2.0
                                -PCI Express 3.0/
Certified for Windows 7          yes               yes
Supported Technologies          -DirectX 11        -DirectX 11
                                -Optimus           -Optimus
                                -PhysX             -PhysX
                                -OpenCL            -OpenCL
                                -DirectCompute     -DirectCompute
                                -3D Vision         -3D Vision
                                -CUDA              -CUDA
Maximum Digital Resolution      3840 x 2160      / up to 2560x1600
Maximum VGA Resolution          Up to 2048x1536    up to 2048x1536
HDCP                            yes                yes
HDMI                            yes                yes