The 285 and 4800 are older generation and not worth the investment. Only the 4000 can be considered.
On the PC side of things the 4000 results in around 10 times better rendering speed than a non-CUDA card. On a MAC, with its lousy threading, dunno.
Whether it's worth it to you is a hard one to answer. I put a 4000 in my mac and the biggest difference is that instead of my CPU's doing the hard work the GPU now does it and free's up the CPU for other tasks.
My 12 core 2.93 Mac Pro plays back XDCAM-EX 1080p footage with colour correction applied just fine without hardware MPE, although it uses about 70% of the CPU power to do it. With hardware MPE enabled the CPU idles while it plays through the same sequence.
If you can play through your timelines now with software MPE then I honestly wouldn't worry too much about buying a new card. If your CPU's are struggling to keep up then it will probably help, depending on what effects and transitions your using.
Exporting is also a bit quicker too, certainly not 10 x quicker though so don't be led by that. It's probably 25% quicker than my ATI 5870 was, again probably due to the CPU's being able to handle the load quite well. It really depends on what your currently using I expect.
You also might find this little video informative, as it addresses a point that Paul made about the GPU taking burden off of the CPU(s):