One thing is not clear for me yet (have no 64 bit CPU for the tests) is what happens with the registers.
In a 64 bit CPU the old 32 bits registers are 64 bits, if you load a 32 bit register the upper 32 bits are filled with zero, until now is OK.
Now if I load this time the same register with the 64 bit prefix and load with some address above 4GB what happens if I access memory as [EAX] and not [RAX]? I shall address the full 64 bit space or the upper 32 bits are considered zero even have a value different from zero?

Stupidity also evolves!