My C55 NB, a part of 650i chipset from NVidia runs at ~46-50C, and I consider that hot enough.
My SB was running at ~62C without the heatsink, now ~50C with the added heatsink, and I consider that hot enough.
65C - 75C is very hot for the chipset, IMHO. Add some extra cooling, like heatsink or even a fan.
I also have a side panel fan blowing on the CPu HSF and the 120mm 1800RPM exhaust fan.