Alright then! Humbly, I stand corrected about my poor speculation without research. It looks like 640MiB has been a perfectly achievable configuration, especially with 2x256+2x64 or such. That is, I must say, a huge amount of RAM. Like, way more than any video game ever specified in HW requirements. What use cases applied that you could use up 640MiB in that era, I wouldn't know!

I remain a bit mystified about why it would be a hard maximum, though. Did such motherboards prevent the user from installing 4x256MiB for a cool 1GiB of DRAM? Was the OS having trouble addressing or utilizing it all? 640MiB is not a mathematical sort of maximum I was familiar with from the late 1990s. 4GiB is obviously your upper limit, with a 32-bit address bus... and again, if 640MiB were installed, that's only 2 free bits on that bus.

So I'm still a little curious about this number being dropped in the article. More info would be enlightening! And thank you for speaking up to correct me! No wonder it was down-voted!