Len, let me clear this? Are x86_64 and AMD64 the exact same thing? Can they run ordinary x86 apps?
Yes, both refer to AMD64. The can both run 32-bit apps.In fact if running a 32 bit kernel it should behave identically to a normal 32 bit x86 processor. If running a 64 bit kernel, 32bit programs can be run. However by default the debian AMD64 port does not support this because it would require 32bit libraries which are a major pain. But most users of Debian AMD64 would have no reason to run 32 bit programs unless they wanted to run certain non-free programs, or one of the few programs that currently don't compile/run right on AMD64 such as WINE and OpenOffice.