79C is just about the highest I would ever want to see my GTX 260s (very similar cards). They can run hotter than that though.. I've seen a couple of benchmarks where the 260 / 460 (can't remember which now) goes up to 95C!!
That looks fine. As long as you don't get any artifacts in games then I'd say that's a very good overclock. Your memory is very high though.. not sure if that'll cause any issues or maybe you just have a freak card