I answered 160-189, but that depends on what you are trying to do. If I am drag racing, I want it as cool as I can get it which is way below 160, but for cruising around I or in traffic I have my fan set to come on at 190 and will cool it back down to 175 and go off. The fan never comes on driving down the road, only sitting in traffic. Even on 100 degree days it never goes over 190. A warmer engine is usually more efficient and that is why the oem's run them warmer, plus it helps to "burn" off [emissions] in the engine.
As far as the too hot goes, I would not want my car to get to 230, but I also do not run a high pressure radiator cap. I have seen engines get way over that and not hurt themselves but not for an extended time period. My Dad has a 57 chevy and it would run hot. We tried all kinds of things to keep it cool and it would not cool down. He has seen 250 with it and never hurt it. The only reason he let it get that hot was he was stuck in traffic in a cruise and had another engine built to go in it.