55 is quite high for idling. I get about 40C on FX-8150 and about 38C on a FX-4150 and I found this post:
http://www.overclockers.com/forums/showthread.php?t=703838
This may also depend on your OS and BIOS power settings, since they will dictate what power saving features can be used. (and also the room temperature obviously)
Remember that maximum operating temperature for your CPU is 61C
http://www.cpu-world.com/CPUs/Bulldozer/AMD-FX-Series%20FX-8120.html
Although I have seen it climb up slightly more than 61 at full load. This also depends on if you have a setting called HPC mode on your bios, if you have this setting disabled, your CPU will throttle itself to reduce power usage if it gets too hot.(or that is what I understood of this option).
Also, I found out that many programs seem to report CPU temperature incorrectly. Especially for the bulldozers. I guess you can trust the motherboard tools more, you should be fine as long as it floats around 60C.
