There is a "sticky" here on this but it it isn't easy to find and it's long winded so I'll post up what I posted at the end of that thread, which should answer your questions on FX CPU temps.
Just an FYI on AMD FX series processor temps...
AMD engineering has looked into the issue and confirmed that the temps below 40C reported by the common temp applets like Core Temp, HWiNFO, OCCT, AMD Overdrive, etc. are NOT accurate. The temps between 40C-70C are quite accurate. AMD engineering has advised that in the next iteration of AMD Overdrive, this issue will be fixed. There is no word as to when the next iteration of AMD Overdrive will be released however.
As a result some folks see below ambient temps when their CPU is at idle. Adding a temp offset to make your idle and light load temps appear normal will produce wildly high, inaccurate temps when the CPU/APU is under load so do NOT add a temp offset - per AMD engineering.
It's worth noting however that the only temp that actually counts is the full load temp. If your CPU is below the 24/7 max temp that AMD specifies for your model CPU/APU then you are fine. Unlike the Phenon II CPUs that tended to like to be below 55C, the FX models run fine up to the temps AMD specifies for each model CPU which is typically in the 61C-70C range for 24/7 use on desktop model FX CPUs.
Building a reliable PC involves more than just assembling the parts. You need to be able to configure all of the BIOS settings appropriately. This can be quite involved and frustrating as it can require a lot of trial and error with stress testing. It is however often the only means to get a 100% reliable PC.