That's correct. Only three options on the MAC, but you really don't need any more.
I recommend the GTX285 for the best performance (and it is the least expensive, ironically). It might be more difficult to find (probably impossible new but there are lots of used ones you can pick-up for a song and a dance).
Here is a blog post I wrote discussing the benefits of CUDA GPU acceleration and the MAX Render Quality. When you see the difference GPU acceleration makes to both speed and render quality you will never render without GPU acceleration again.