Re: AMD Dreams
The big gain in the move to x86_64 was the ability to directly address more than 4G of RAM. At the time ('05?) that was becoming important. IBM had it in POWER and it really made a difference in DB/2.
There was a delay in bringing 64-bit addressing to x86. As you noted POWER had it before x86, as did SPARC (1994), Alpha, and Intel's own IA-64 (Itanium). But that was intentional by Intel.
Back then, as now, Intel loved to segment the market, to be able to push more SKUs with slightly differnet feature sets to better monetize their products (read: screw over the customers by ripping them off). Now there are different editions of essentially the same Intel CPU that have different memory support, not different types of memory, different sizes of memory.
Take for example the current 8280, 8280M and 8280L, which are essentially the same CPU. They are all 28c/56t, 2.7GHz-4.0GHz, 6 channel memory, 205W TDP processors. The only difference is 1TB, 2TB and 4.5TB respectively RAM support, costing approximate list price1 of $10k, $13k and $17k. Purely so Intel can charge a premium for greater RAM capabilities, there is no technical necessity in the different RAM limits or cost to Intel in supporting 1TB vs 4TB2.
In the early 2000's Intel was intentionally segementing the x86 market into low-RAM 32-bit x86, and if you wanted high-RAM 64-bit you'd go IA-64. It took AMD's AMD64 to break that, and Intel adopting AMD64 as x86-64 in its own processors to counter AMD was a major setback for their IA-64 (among other things, like the concept never working as noone could get a proper optimizing compiler to work well on it).
1. no-one ever pays list price. It's the starting point for negotiating on price. Anyone buying one of these processors will pay substantially less, like 20% less. Anyone buying a significant volume, say fitting out a data centre with scores or hundreds, will pay 50% or less in all likliehood. But the ratio difference between the different memory support models remains in effect.
2. As evidenced by AMDs server-processor model where every server CPU can support 4TB of RAM. There is no RAM capacity segmentation.