The Opteron and the FX CPU are equal and thus equally fast. The only difference is that the FX lacks one pin (and so it can operate without ECC memory) and can’t be used in multiCPU systems.
The difference between the two consumer CPU’s, the Athlon 64 and the Athlon FX series differ quite some in performance. For now, the only difference between the FX and the 64 is the capability for dual channel memory (FX feature). In the near future, there will be versions of the Athlon 64 with less cache on die (lower performance, lower price).
For now, the fastest AMD consumer CPU is the FX 51. It performs about equal to the Intel P4-EE @ 3,2Ghz, but comes for about â‚¬200 cheaper. Most probably, the AMD CPU will beat the Intel P4-EE pretty soon, as more X86-64 programs will come available (those’d really make use of the potential of this chip).