I'm pretty sure that AMD does not make a 6870 chip that supports 4GB onboard ram, unless you're talking about system ram, which is a different story.

To the person above me with the Geforce 560, I'm going to recommend lowering your resolution to 720p and possibly disabling vsync when you play. Your avg framerate is in the 30s, but like my situation, when I enter a high traffic area and especially during a FATE, the framerate will drop down into the lower 20s/high 10s, which makes the game not run smooth at all. A 660ti or even possibly a non ti would have put the avg framerate of your system at about 60 using high settings. Most people actually experience 60+ using maximum settings.

I'm not saying to go buy it, but I am just showing some general performance numbers as you were looking into. I've run into a similar delima with a 6870 card I recently purchased, which is close to your 560 in performance, where, for me, its definitely worth upgrading to a faster 660ti especially with the used ones from eBay going for $200 now.

I don't think you will notice much between high and max settings, however.