Is the Athlon 64 FX going to work in the socket 940 or is it the chip that gets the socket 939?
And to answer your question, I think it would speculation to say this, but it should do well as long as it can maintain good data bandwith's and latency. Its IPC will eventually be augmented by software written for 64 bits and then it will perform well.
If your question is, will it be faster than a similarly priced P4 or Xeon based system (or even a Prescott based one). Maybe depends on the configurations of both systems. And Prescott is as big an unknown as the Athlon 64, so ..........??????
Just guessing.