Mine has been recently cleaned and operates at 59°C northbridge and 48°C heatsink under load (doesn't vary much with load though). I guess dual-cpu might run a bit hotter. Sometime ago the temps used to climb around 80°C causing the CPU booster fan to run constantly at 1000-1100 RPM (well I guess it was the cause). Cleaning the whole CPU tray did the trick for me (it had never been done for 4 years) but some had issues with the northbridge heatsink plastic retainers that can break over time as described in the link. If you get no reading from the CPU booster fan you might want to check if it's working as well and properly seated following the CPU upgrade (the one that's inside the CPU heatsink, I don't know how this translates on a dual-cpu, are there two readings like BoostA and BoostB ?).
In general chips don't like to operate much higher than 90°C as it increases the chances of failure/malfunction. Most manufacturers seem to design their systems so that the chips temps does not exceed 83-85°C. Most chips are designed to operate at 105-115°C maximum. It seems your northbridge is right on the edge and I wouldn't recommend leaving it as is.
