The onboard 2MB ATI Rage IIc is considered to be a 2D "accelerator" and doesn't cut the mustard running anything 3D accelerated, it simply doesn't have the texture memory to do anything. I think there were versions of Descent and Mechwarrior II that shipped with the 6500 but but provided very basic acceleration not much better than software rendering - if you can find these, let me know!
So, for RAVE/OpenGL games you ideally need a PCI 3D accelerator you can plug an external display into. My experience with such cards on a TAM (same ATI Rage IIc) is that ATI card drivers don't play nice with the older Apple IIC drivers, so you can have either one good card and poor performance on the other. I'd look into a 3DFX Voodoo 1 or 2 in a 5500/6500 machine, as it doesn't conflict with any drivers and gives you that proper 3D accelerated game experience that early ATI Rage chipsets were terrible at doing.