Well, supposedly, the FX-57 has the power to match a 5.7GHz processor, but I don't think one exists from Intel. Nor do I think Intel has a 4.4GHz processor, which the 4400+ would initially match. Anywho, basically any AMD processor will give you extreme over clocking capabilities, as every AMD processor is extremely underclocked to begin with - because of the heat they generate. If you do decide to overclock any AMD processor, I suggest you get a liquid cooling system, just so that you can overclock (and overvolt) it all you want without frying it with extreme heat. Though do be careful with overvolting, because you could fry the CPU, not because of the vCore, but because of the voltage tolerance of the static RAM in the processor.
And if you are going for an X2, I suggest ya get the 4800+ because it's just that much more powerful, but it also has the dual core.