I have no idea what divyanshu is on about ?
As those are professional cards im assuming this is for professional 3D modeling or some such ?
I'm not an expert on graphics programs but i believe that the important thing is the actual size of the frame buffer, more being better here and not the speed of the Ram as would be the case for gaming.
As such the 2GB of ram would benefit you more as it allows you to hold more info in the frame buffer which i believe to be the important thing in rendering work.
Hopefully another poster can clarify this for me ?