Does that not depend on the rest of your system? I think it might be worth considering 4gb 680s in sli if you are looking for 3d multi-monitor gaming. Although I have not noticed any issues (I am only using one monitor currently though)
At 3DMark 11 performance setting, which runs at a lower resolution and quality, the SLI 690 scale to 34%, meaning the CPU is the bottleneck here. Extreme setting brings the difference between one GPU and a SLI config to 74%.
I was going to get a i7-3930k OC to 4.6GHz to accompany them, still thinking of whether to SLi or not, some people notice big improvements while some only notice small improvements, I will EVENTUALLY be using a 3 monitor setup, as of now it will be a 2 monitor setup.