The difference between the 64 and 64x2 spec is that the x2 variety is effectively 2 cpu's on one die. I run a dual core fx cpu and it does give a boost in performence, particularly in games.
The 64 refers to 64 bit instruction set support. There are 64 bit versions of Windows available, but hardware vendors have been slow to provide driver support for the 64 bit enviornment. The 64 bit cpu's are, however totally compatible with 32 bit Windows which is what most of us are running at present.