The GTX 285 has 1GB of GDDR3 and the Radeon 4870 has 512MB of GDDR5. How much of a difference does the type of RAM make in performance? A naive point of view would think that GDDR5 is better since it's newer.
The amount of ram has absolutely no effect on gaming performance unless the game requires more than 512 MB's of ram. There are a few games available which do, none of them available for Mac OS X.