Hello
I recently tested the ATI video cards. The problem is that even with 1MB of vram, graphics modes higher than 640x480 are not supported.
I know an 800x600 driver for win 3.0 exist, but until now I only tested the card in DOS mode with the Vesatest utility and the VESA driver for ATI (vvesa.exe).

BTW:
1 I know a bug existed in Pcem in 2015, but I don't know if it's the same bug. https://pcem-emulator.co.uk/phpBB3/viewtopic.php?f=2&t=304&p=1871&hilit=ati+800x600#p1871
2 It seem to be a bug with ATI cards because I tested Vesatest with Tseng ET4000 and the Tlivesa dos driver and worked correctly