It really depends on your cpu. 3,8 - 4 GHz would make a difference in performance. If it can do it without raising voltage too much, it's worth the shot. With water cooling you don't need to worry about the heat as much as people overclocking on air and even 1,35 Vcore shouldn't be too hot (I wouldn't go more than 1,375V). Maybe you could push it even beyond 4GHz. Google some guides on OCing 960 and see what results people get. Unfortunately it requires a lot of testing and tweaking various settings to make sure it's rock solid.
As for GTX690, if it's for free then there is nothing to think about. Just take it
And save money from sold 7970 for your future build, you will buy some monstrous Ivy Bridge-E then.