Am I amissing something here 800x600x256 Workbench using AGA?
I can run 800x600x16 on my ECS A500 with Indivision (after updating to 1.10 firmware) and the SuperPlus drivers. It works quite well, was messing around with it again last night, but after opening Workbench and DPaint both at that resolution I think I only had about 100KB of chip ram left.

I've seen forum posts that other people are running 1024x768x4 on ECS machines with the HiGFX drivers, haven't tried those yet.
Am almost certain AGA machines can do either resolution with 256 colors and the right drivers, but think there's still some bugs to be worked out of the Indivision for those models. :-/