What version of Photoshop and Illustrator are you using? Some versions can take advantage of Nvidia's CUDA support and will let you off-load some rendering to the GPU (off of the CPU). If that is the case with what you are using, the GT640 would be the better choice.
It really depends on the versions you are using.
Go here for more info:http://helpx.adobe.com/photoshop/kb/photoshop-cs6-gpu-faq.html