The core usage is not unusual. Mine does the same thing, one being used much more than others.
Those temps aren't bad if your system is overclocked, but not great if you are running 2.40GHz. The Arctic Freezer 7 Pro is at its limit with a quadcore. 50 degrees isn't going to hurt anything though.
I had a Thermaltake Big Typhoon on my Q6600 that had the same problem. I was running 55 degrees at 2.40 GHz. Pumping up the voltage and speed to 3.20 GHz, it couldn't stand the added heat and temps went to 70 degrees. Switched to a different cooler (in my case a Zerotherm Nirvana 120) and temps dropped dramatically, to around 50 degrees while running Prime 95 for over an hour.