The current AMD FX CPU's are special in the sense on how their architecture works. AMD Advertises their CPU's by core count, however, this is not the true case. Their CPU's have something called Modules ('core') with two Integer Units inside (threads).
The FX-4XXX CPU's each have two modules (2 Modules, 4 Threads). FX-6XXX CPU's have three modules (3 Modules, 6 Threads). FX-8XXX CPU's have four modules (4 Modules, 8 Threads).
These CPU's tend to excel greatly in heavily multi-threaded applications but suffer in single-threaded applications due to how the architecture works.
Long story short, it's somewhat similar to Intel's Hyperthreading in a sense but on a hardware level.
FX chips are bad. I'd go with a 6 core phenom over a any fx quad / six core.
They don't have full physical cores, which is not very good. Phenoms have all physical cores, which make them the best line of cpus you can get from amd. Under certain circumstances, the 8 core is okay, such as HEAVY multitasking, and rendering and such.
Actually, the current PD chips aren't too bad. In most cases, they actually beat out their Phenom counter parts (finally). But are they worth the price/power consumption? No. Intel definitely has AMD beat in that respect.
On a side note, I would also recommend a Phenom II X6, but keeping in mind they're VERY hard to come by nowadays and often come with a hefty price tag, it's best to stick with Phenom II X4 965's or one of the new Piledriver FX chips.