I do agree that 2GB is enough but again be careful.
Last year for most 580 or 570 owners 1.5 GB was pretty enough as well but I doubt 1.5 can handle surround system.
But the most important thing now is the upcoming next gen console ports.
Though I am pretty sure that 2GB will still be good enough for single monitor, I have my doubts over surround. However 4GB will only shine if SLI or Triway SLI is involved. I would never recommend anyone buying a single 670 with 4GB if they will never SLI it in the future. THough if they are fine with paying a bit more, that is always ok too.