ok before alexh pops in I'ld repeat his line, the best accelerator for the A500 would be the mythical
Viper 530. But really, this card is very uber rare so just forget about it. A
GVP A530 would be nice, and they're not so rare.
Derringer 030 and
VXL-30 are other decent boards that you can have a chance finding. But all these boards would cost you a good lump of money, so if you don't really need it I wouldn't bother.