You may or may not get a stable OC, its trial and error. If your not confident start high and lower it. For example start your vid at 1.4 and lower it until your system is unstable.
Cooling wise get something with good CFM. A very nice fan is the xigmatek XAF series very high airflow and can mount on a 120mm mount. Look it up. High RPM fans are very loud, personally I have the XLF series, 63CFM and nice and quiet. I have 6 of these running.
You want either the same amount of intake as exhaust or more intake. Helps pushes warm air out and reduces dust. Coolermaster sickleflow are also a good choice, 69CFM but quite loud.
With your board I think there is fan control on the headers?
As for overclocking depends, not all boards and chips are the same. My motherboard is based on the phenom and from reviews it is apparently awful at overclocking the fx. The max oc I have seen people get is 4.4ghz. AChips and boards are different.
I think the fx-8120 can clock higher for some reason, but it doesn't mean your chip can run 4.0ghz+ at 1.2v (example). It may need 1.3, 1.35. Either way 4.0Ghz OC is very nice on the fx, anyhing above is just a bonus really.
As MrPerforations said go for whatever you feel comfortable with. You aren't going to damage the chip unless you undervolt it or overvolt it by a lot, and run high loads at 60c+ for prelonged time.
Keep your temperatures under 60c, keep your voltage (vcore) under 1.4 and you'l have a nice oc that won't degrade the chip like crazy.
The only thing you need is patience. I just found out my chip prefers a lower voltage, and that it OCs better using the fsb. Overclocking takes a lot of time and a lot of playing around, it can take weeks to get the best OC. Just keep to the guidelines on temperatures and voltages and you'll be fine.
System freezing, BSODs, no startups are all outcomes of an unstable OC, and is a protection feature so really it's a good thing. Most of us have pulled our hair out the amount of times we've had a BSOD and had to clear the CMOS.
So yeah, we're all here to help you a long the way, we all have stuff to try out and learn with the FXs, that's why their my favourite chip
Have fun overclocking, don't hesitate to ask questions, we all start from somewhere