GPU for now, R9 370 is what I would look at or GTX950. Anything stronger will be CPU bottlenecked even if you reach 4GHz which is like max those CPU's get.
You should be able to push a 960 on a 965, Im running a 760 without bottleneck and still have a little room to go. Clutch says hes running his 960 with a 965 fine as well.
Overclock, essentially you make the processor run faster than the factory intended. Every processor OCs a bit differently, some processors hit a limit (mine is 4.22) when others done. (I believe user Robert Cook hit 5GHz on his 965)
Hey, so im thinking about upgrading to the gtx 760 now, ive oc'd my 965 up to 3.9 ghz so far, with the specs i've given could i upgrade to the 760 without a hitch?