I haven't overclocked my 965 since it's doing everything that I need it to. I've played Fallout 3, Borderlands, Unreal Tournament 2004, Orcs Must Die, Modern Warfare, and others that I don't remember.
Personally I'm sticking with the 965 until the day it either dies or bottlenecks but I don't believe either of those events will happen. If anything, I would upgrade my 6850 before I upgrade my 965 since how well your games look and perform depends more on the GPU than the CPU.
So i have an athlon II X3 which is bottlenecking my 6850 so i think that a large difference would be with a 965 BE since i hear it won't bottleneck a gtx 670 to which i was planing on upgrading