A graphics card can allow 24-bit color and faster 3d graphics plotting than the Amiga chipset. The Amiga graphics on-board are good for games and contain some forms of graphics acceleration that most computers don't have. They are slow by todays standards and you only get maximum color graphics in low resolution on the A3000.
The A1200 has better color than the A3000 and the AGA graphics chips are twice as fast as the A3000 since the A3000 doesn't use fast-page memory. Also it will probably be easier in the future to get an expansion board for the A1200 such as the Mediator. It is usually cheaper to buy PC-style cards to plug into the Mediator than it is to find Zorro 3 or Zorro 2 cards to plug into an A3000.
Still, an A3000 is faster than an A500/2000 since it has 32-bit memory access in the graphics chips but not as fast as an A1200/4000's AGA chipset.
