Get the 2GB one, and "super super clock" it yourself. Overclocking 600 series cards are a breeze as long as they have good cooling or you don't mind a loud fan.
So which version of the GTX 650 would you recommend me? I am trying not to go above $150 if that would be of any help. Also, should I get the normal version with 2GB, or the Titanium version and 2GB?