To be honest, 1024x768 under AGA is possible provided you don't mind awkward, possibly monitor damaging settings, eyebleedingly slow refresh rates and having no spare cycles left to the Chip RAM for the CPU to actually render anything.
I used to try stuff like this all the time and it honestly isn't worth it. I ended up using 1280x1024x16 autoscrolling on a 640x512 display with a flicker hack called MagicTV for a while, but did the sensible thing and got a graphics card.
