There are various ATi Radeons or Rage cards to pick from on the GPU front.

These are most likely the most expandable Mac's that will ever exist really (9x00). Nice find! Nice thing about the 9600 is how the case is not 100% made of brittle Spindler Plastic.

Mac OS 7.6 will be hyper-warp speed on an upgraded 9x00. I would give it a try before OS 8 or 9, just for fun.