Hi there,
The temps you are getting are nothing to worry about. I ran this board for ages and really abused it with lots of testing and reconfiguring and, when I finally upgraded, I gave it to my son who has been using it for almost 12 months now.
The NB Temps will, of course, be dependent on your hardware and what you are actually doing on the system. If you have a decent Graphics Card and are watching video or playing games teh Temp will rise. It will also get hotter if you are overclocking the CPU or RAM.
You can remove the NB and replace the thermal paste, if you believe it will help and, as long as you are careful, there shouldn't be a problem. Warranty for motherboards is 3 years from date of manufacture so you will have to consider if the possible benefits of removing the NB Heatsink and replacing the thermal paste is really worth the possible loss of your remaining warranty.