To begin with, I have worked with x86 asm code. I have read on architectures. I have the very basic knowledge as to how digital logic and other cpu circuitry (ALU, Micro code, etc) work.
That's a valid point there. The way I see it, is that if coding can be done just to tailor to register width extension, you can create a standard coding path which works on any 64-bit x86 CPU, and then go for individual core-specific instructions to optimize even further.
There is no similar "coding path" (whatever that means), between an AMD64 cpu that supports both x86 32bit instructions and another 64bit cpu at the moment. Itanium can run emulated x86, but does not natively support x86. There is no "hybrid" x86 chip from intel at the moment that is both "64bit" and has x86 ISA support.
Secondly, the difference between 64bit and 32bit in reference to cpu architecture isn't as simple as 64bit registers vs 32bit registers. ALL computational components of the CPU (ALU, FPU, etc) must be modified to support computations on 64bits rather than just 32bits. If that is not done, then you are wasting 32bits by just performing the old 32bit instructions.
Thirdly, the only way to have compatible "coding paths" (again, this term in reference to what i believe you are talking about is being used in completely incorrect context) is to have compatible ISA (Instruction Set Architecture). x86 is the ISA your Pentium 4, Athlon XP, and even 386 is compatible with. MMX, 3dnow!, SSE, and SSE2 are all extensions on x86 meaning they are optional and can optionally be supported in x86 code. AMD64 ISA (formerly x86-64) can be treated as a new ISA, NOT an extension.
Therefore, the only way to take advantage of the 64bit-ness of AMD64 processors is to recompile or produce code in AMD64 ISA, not x86. Although the instructions are similar between x86 and AMD64, you can't simply take an x86 program and start using AMD64 instructions and expect that mix of x86 and AMD64 code to work on old processors as well as new ones. It's either x86 code or AMD64 code, not both--more specifically x86 compiled code when recompiled as AMD64 code (no instruction changes) will work, AMD64 + x86 "like" code compiled as AMD64 code will work, however, x86 + AMD64 code compiled as x86 will not work--it won't even compile.
I could continue on, but I think that is sufficient to illustrate the point.
I may be wrong, I have no programming knowledge.
There's a difference between speculation and saying something utterly wrong. If one speculates, then there is little or no evidence at the time of writing that immediately proves the statements false or true. x86 and basic cpu architecture has been around for more than a decade.
So to conclude, what you have said makes no sense.