On my Panasonic Toughbook CF-M34, with a SiliconMotion LynxEM(SM710) video chip, this is still a problem on Debian 11. I get the same issue "Not enough video memory for the configured screen size (800x600) and color depth."
It seems no amount of xorg.conf experimentation will make this error go away. Even setting the VideoRam manually doesn't help. 800x600x16 should fit within 4MB(it works just fine in Windows 98SE).
