Radeon 7000 or Radeon Mac Edition (7500). Both work with System 7 (with these drivers, work fine under 8 and 9) through to OS X. I can't find ATI's official old drivers anywhere.

For older/cheaper cards the the Rage 128 is probably as low as you want to go.

Or, you could get one of the old Voodoo 3 cards, drivers here

I'd go with the Radeon 7000 FWIW.