Stick with the 680 m8 there about to pull away from the 670s
With the next driver updates from what I've heard
Can't see how thats going to happen as they are the same architecture just with one processing unit fused off on the 670.
IMO 6 series are primarily limited by memory bandwidth more than processing grunt so the 670 with the same memory size, speed and bus makes good sense. Especially some of the non-referance models that routinely allow for good memory overclocks.
The GB WF 670 is EPIC. Recommended.
