Have to disagree with Brett on this one. The only advantage of going to the open system is getting rid of the sometimes leak prone plastic bottle.......there is NO advantage in cooling. From a purely technical standpoint, a closed system is supposed to be slightly better than an open system for pure cooling ability. I live in Bakersfield, CA, in the central valley, where it regularly gets 100+ and I drive up long hills to go anywhere to wheel.......and I'm still running the closed system. I've had a small amount of trouble with overheating three times (in 9 yrs). First time the stock radiator was partially plugged, replaced it with a new two core. Second time the now four year old radiator was partially plugged, had it rodded out and back flushed the motor (which I should have done the first time). Third time I had trouble getting a good cap on the plastic bottle, went through three of them, until a tech at the Jeep dealership gave me the part number to a Volvo cap that is a much better cap, bought it and no more problems. My plastic bottle cracked and started to leak one time, I always carry a spare, and it took about 10 minutes to change it out.
What is too hot is when the temp gauge is in the red, if it's not in the red you are OK........not good, maybe, and at risk of it getting into the red, but OK. Under a good load, on a hot day, it can run at 230 with no ill effects, and even hotter. What's not good is if the water boils inside the cooling system, actually in the block, in which case the cooling ability pretty much stops and the water escapes the system due to the high pressure........from the pressure relief in the cap on an open system and through the escape hole in the cap of a closed system. Two things increase the boiling point of the water, anti-freese/coolant and pressure. The system runs under pressure so the boiling point is increased allowing hotter temps without boiling. When the cap is bad (typical on a closed system) the system can't hold the pressure, so the boiling point is lowered and water escapes the system. The coolant also inhibits corosion, and corosion is what plugs the tubes of the radiator, diminishing the cooling abilty. So, a certain amount of coolant/anti-freeze is good, and a good cap is mandatory.
Usually, running continually at over 220 means the system should be checked out, but it doesn't necessarily mean you'll find anything wrong based on the conditions. Running that hot more than breifly means you should check things out when you get home, but it doesn't mean that anything bad is happening to the engine, unless it gets into the red. BTW, the auxilliary fan comes on at 210*.
