AMD64 is a name of 64-bit extension for x86 architecture. There are lot of AMD CPU which support AMD64: Athlon 64, Athlon II, Sempron (not for S462), Phenom, Phenom II, Turion, Opteron. These processors can have various core number (1, 2, 3, 4, 6), which usually denotes by X suffix: X2, X3, X4. Opteron has its own model numbers, without X suffix.
About differencies: there are no general differencies between those CPUs, if we are speaking about the number and width of registers. Some models can support specific SSEx, virtualization and some other extensions, but this situation is usual, because some CPU models are older, and some are newer.
If CPU contains several cores, this allow to execute more tasks in parallel, but single task (more exactly: single-threaded task) won't any speedup because of this. It is very complicated work (for the programmer) to get performance boost using multiple CPU cores.
Thank you very much.
However, I haven't been precise; I was talking about Athlon 64 and Athlon 64x2, but your post gave me the answer the same.
There is no such thing as "AMD64x2".
Both "Athlon64" and "Athlon64 X2" processors use the same instruction set architecture, called AMD64.