What is the highest end GPU that I can use with a phenom 2 x4 965 without getting too ambitious in terms of overclocking?
Depends on the games and the load. Each game will require a different mix of resources. In general, I'd say a 7870 would be a good place to stop.
you shouldnt see much if any bottlenecks with a 7970 or 680
that cpu is still very strong