As for the RAM, I can confirm 256MB is possible on an UM8881F chipset. 10 years ago someone gave me a small 486 with a tiny motherboard with that chipset that had been upgraded about as far as I'd ever seen a 486 upgraded (AMD Am5x86DX5 "PR75" (=133MHz) CPU, 32MB RAM, 8MB SiS 6326 PCI video card) and had still been in desktop use (running WIn98SE) up to that point. But at the time I had *a lot* of components lying around, so I decided to go one better.
First I upgraded it to a Voodoo3-3000, then added more memory. And more. And more. It only had two 72p SIMM slots, but every time I gave it a bigger module, it detected it and happily used whatever was in there. Eventually I was running 2x 128MB SIMMs. Unfortunately I hadn't bothered to check whether it could cache this whopping 256MB, but it certainly worked.
I then tried to install Windows XP. This of course failed because XP requires at least the 586 instruction set (and the 5x86 is, despite its name, a simple 486). So I replaced the CPU with an Intel Pentium Overdrive PODP-83. That worked, and about 12 hours of installation later I had a working Windows XP desktop and confirmation that the 256MB was available to the OS.
The 128MB SIMMs were double-sided SIMMs, but I don't remember the exact chip config. Assuming there were 16 chips on them, they would have been 16Mx4 chips.