AM3+ is the required socket for AMD's Bulldozer (older) and PileDriver generation FX CPU series.
Do not buy a "vanilla" AM3 motherboard. The FX series CPUS will not work with that older socket. It must be AM3+.
Additionally, pay attention to if the motherboard supports 95w TDP CPUs or 125w TDP CPUs. Buying a 95w TDP motherboard will probably be a little cheaper, but it also means you will not be able to install a higher end CPU.
For example, if you want to install a FX-8350 in the future, then you need to buy a 125w TDP motherboard. Otherwise the FX-8350 will simply not work.