I'd look at getting a new rig. If your focus is gaming and not professional design, get an AMD CPU; much cheaper than an i7 and it'll leave you with plenty of money for the graphics, which is the biggest factor in determining your FPS. Right now getting a high-end card such as the HD5850/GTX470/HD6950 is a waste.
The E6850 can still be used to play games. You do not need to upgrade if you do not want to, but you can get better gaming performance if you do. I read an article that generally compared dual core to quad core CPUs in various games. Generally speaking, the average performance increase was about 15% basically because games were at most only using 3 cores.
I recommend upgrading the card first since that is the single best way to improve performance. Without a budget it's kinda hard to determine which card is best. If the performance is lower than you want then you can upgrade the rest of your system.
Of the three video cards you listed, the GTX 470 offers the best performance.