AMD64 == x86 with 64-bit extensions and 8 more registers.
No, AMD64 is not x86 with 64-bit extensions. AMD64 is a full 64-bit architecture that happens to offer compatibility to existing 32-bit x86 code.
Your use of the phrase "with 64-bit extensions" is FUD in that it may be taken as meaning that the 64-bittedness of AMD64 is incomplete, only offering extensions in some of the areas needed to move to a full 64-bit architecture.
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