IMO FX series are not true quad, hexa, octa cores, they have a different architecture from any other chip, they basically have 2 modules with 2 cores sharing each module (for FX4100) and 3 modules with 2 cores each and so on.
Current Windows cannot fully utilize this architecture (Hopefully Win8 can) performance is still not on par with 2 cores with 4 threads from intel.