91C is too high for the CPU for long periods of time. Average working temps under heavy and full load are in the 60 to 70C range and they are absolutely fine. Although we all like to stay below 70 if possible.
For your rig, 48C is fine but I doubt it's under full load, try the IntelBurn test and keep your eye on the temps to see what is that max you reach.
You can download it from here.http://downloads.guru3d.com/IntelBurnTest-v2.3-download-2047.html
Use HWMontior to keep your eye on the temps or core temp.
Download from herehttp://www.cpuid.com/softwares/hwmonitor.htmlhttp://www.alcpu.com/CoreTemp/
When the rig is idle you should get temps in the range of the ambient temps of the place you are in, plus minus 10C.