The OCModShop crew lays out the differences between AMD64 & EM64T in a handy crib sheet. Good for a quick reference.

"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."