While some of the theory involved in here may interest some, there are some questions that are on people's minds.
Will a 64 bit CPU run a standard, 32-bit program on a 64-bit version of an OS? Yes it will. 64 bit systems are backward compatible with their 32 bit system counterparts.
Will a 64-bit OS run a standard application on a 64 bit processor? Again, it will. This is due to the backward compatibility element.
Can I run W2K and WXP and Win7 on an 64 bit CPU, and use old software? Yes, a 32 bit OS (W2K, WXP, Win7) will run on a 64 bit processor. Also, old software will be able to run on a 64 bit OS, but it will not be efficient.
Finally, the difference between 64 bit and 32 bit Windows 7 is how the operating system coordinates the instruction set from programs before the CPU receives them. The CPU will process instructions in a cycle, from start to finish, and a 64 bit instruction is more efficient. For example, a 32 bit instruction may take two cycles, whereas a 64 bit instruction can perform the same operation in one cycle.
See Also: The Difference between Windows7 X64 and X86