Jumping from a 460 to 670 will be a massive jump in performance. I had a 6970 before my 670 and even then, the difference in performance is night and day in the newest games out there. Depending on what you'll be doing on the second monitor, the 4GB version of the 670 can either be worth it or not worth it at all. Personally, I would stick with the 2GB version and save a bit of money to got towards getting a higher end 670 such as the Gigabyte Windforce version or MSI Lightning version, which are two of the best you can get. As I always say, it's all up to you and your gut.