Over the last year, CPUs extending the IA-32 instruction set to 64 bits have hit the streets. There are two incarnations of it, AMD's AMD64 and Intel's EM64T. While very similar, there are in fact a few differences between the two. Not huge differences, but they do exist.

First let's discuss what CPUs have what capabilities. Here's a brief
list:

For the crib sheet to that, 64 bit Integer calculations are one of the things that make 64 bit CPUs so great - they can make 64 bit calculations in hardware rather than emulating it with a bunch of 32 bit calculations. The address space is the logical address space the CPU can handle. The AMD64 and EM64T CPUs do not have a full 64 bits of address space, as they won't need it for a while. They use 64 bit addresses and truncate them internally to save on die space.