Are you trying to speed up render time? Or editor refresh rate? If your trying to speed rendering up, your graphics card won’t do much what you need is a fast processor and LOTS of ram. For editor refresh rates, Nvidia is really good or ATI, which one will depend on personal preference. I would recomend the Nvida Quadro, over the Geforce, since the Geforce is more oriented towards games. The problem is I don’t think you can get the Quadro for mac. You could try ATI and see if any of there work station graphics cards will work with a mac. Otherwise just use the Radeon X800 or Nvidia 6800 DDL both will work just fine.
Best of luck,
Simon Carlson-Thies