The original 64KB 5150 motherboard (4 banks of 16KB each) supported 512KB AST and other 3rd-party option cards, but carried ROMs that had a total system limit less than 640KB. The second gen motherboard supported 4x 64KB, of 256 KB on the motherboard, and 640KB of main memory overall. My recollection is of some number like 512KB + 32KB, for a total of 544KB, but it could have been 512KB+64KB, or 576KB; STILL not 640KB. I remember this because I once had to replace ROMs from gen 1 motherboards so I could get some machines up to the full 640KB memory available.
What I remember more, however, was how fast IBM's original expectations for the PC were surpassed by people using its relatively open architecture to do far more with it than IBM had planned (or anticipated). In 1980-81, few at IBM (or anywhere apparently) could conceive why one would want a PC with more than 128KB (64+64). By being open to change, the PC went quickly from that early 8-bit kind of view to one that would lead to a revolution in business and home computing. You can say what you want about PCs and Windows, but this is being typed on a garden variety home built PC vastly more powerful in every way that that distant ancestor. It has also had RAM, storage, and P/S upgraded over its lifetime (5 1/2 years thus far). Still useful, and more importantly, still usable with new OS versions (started on XP, moved to Win 7 when stable); and while I like having long HW and SW lifecycles, the point is I am not stuck with it as it was - like I am with my DVR (which is just a specialized Linux appliance really).
Thus I do wonder what the last 30+ years would have been like if the "Apple appliance computing" model had been adopted by IBM in 1980-85 instead of the more open one used for the PC / XT / AT? Even though it came out in 1984, the first gen Mac was ridiculous - a completely closed Moto 68K-based mini-workstation with an "8-bit machine" memory limit. It **could** have been built with a removable bottom plate and enough memory sockets for 4x 64KB - but only 1/2 populated (an expansion capability similar to what is now available for its distant descendant, the Mac Mini). But it wasn't - you had to physically upgrade your 1st gen MAc to get decent memory: to 512KB, aka the "Fat Mac", and then upgrade again to get a hard drive in the Mac Plus. Or you had to resort to strategies that would void your warranty (e.g., the hardware equivalent of a "jailbreak"). This Jobsian approach to evolution - via sales of more hardware - should sound familiar to Apple fanbois everywhere at this point (and why I opted not to buy this year's version of the iPad Mini, but wait for - GASP - the one with the proper CPU, camera, RAM, and screen).
Ancient history? Not really. At least two current trends (1) "wirecutters' and (2) cloud computing are going to see this "open architecture versus closed appliance/service" competition played out yet again. (A third may be iOS versus Android smartphones.) Overall I am still optimistic that on balance openness will lead to innovation that will be beneficial and also not necessarily anticipated by those who want everything tightly controlled for their own profit. This doesn't mean however that appliance advocates won't put up a good fight.