According to Fudzilla, the new Radeon HD 4890 that will be unveiled on April 2nd, is more than an overlocked Radeon HD 4870. Important optimizations have been made on the GPU. As a result, it would heat less, while draining less power when idle than the 4870. So computing cores have been modified to be even more efficient.
As Apple is not used to make frequent update in its offer of graphic cards, and usually synchronize the release of new model with the upgrade of the Mac Pro, they will most likely never be any Mac Edition for the Radeon HD 4890
