The GTX 760 has much better value. You could likely alleviate any bottlenecking with a solid overclock. And as the poster above me has stated, it would be a HUGE jump from a 640.
Alright that's good to know, ill choose between the 760 and 660 ti when i got some money in my pocket and i'll have to look a little more into overclocking later too. I will eventually upgrade my CPU to something better but for now i'm more focused on a GPU right now.