640KB was the max RAM. My first PC was the same configuration, except I had a 32MB drive. Oh! And towards the end I had both a CGA and Hercules card so I could have two screens.
It looks like the model 5150 didn't come with a hard disk, and the only official support for that was through the IBM 5161 Expansion Chassis. I think the author would have commented having an external drive for a PC, so I think it's safe to assume at least an XT equivalent.
(I've been searching for images of the chassis but to no success. I've never seen one, nor heard of it before today.)
And in the other direction, a PC Jr (also an 8088) maxed out at 128 KB, according to my references, though Wikipedia says there were third-party extensions to 736K. In any case, the Jr doesn't support the 8087 math coprocessor, so that's not what the author has.
It says it's a 5160 on the back by the power supply. The HD is, in fact, internal. And there's a phone jack on a card in back that looks newer than the rest, so it might have some kind of modem, though I have no idea how fast that is.
I also have no idea if it still works. It was covered in quite a bit of dust and I wonder if the HD won't be stuck after all this time.
I had a PC jr. The base model, which I had, came with 128k of memory, a single 5.25" inch floppy, and 2 cartridge slots. I booted DOS via a cartridge. There was an expansion unit you could purchase which had a 2nd floppy (very handy!) and I think it expanded max memory to 640k (but might have only been 256k).
