It depends on what resolution you're playing at but like I said, I wouldn't go slower than a GTX 680. Go for a 770, 780, 780Ti or Titan depending on your budget.
Here in the UK you can get a 770 for ~£240, a 780 for ~£360, a 780Ti for ~£520 or a Titan for ~£800.
2GB is fine for 1080p. 3GB for 1440p and 4-6GB will be fine for 4k, although you'd probably need to go SLI to get good frame rates at that resolution.