An 8 core FX wouldn't be a bad choice, however, with ZEN (probably? Who knows with AMD...) around the corner, you might want to wait. 40% more instructions per clock cycle=big performance improvement.
Have you considered and i7? Or do you prefer the cost savings offered by an 8350 or similar CPU?
Keep in mind there are 4 core 8 thread Xeons that will fit a standard board and they are quite cheap. That might be a better option for you.
Also, while the 8 core AMD cpus are cheap, the motherboards they need to run properly without overloading the power delivery section of the board causing premature board death cost about $130 and up. You could mate a Xeon with a $60 board. Just something to keep in mind.
Also, the AMD AM3+ platform is limited to DDR3 and PCIE 2.0. It also lacks many modern connectivity ammenities in most implementations.