PC Mag published a test of the new high-end
iMac clocked at 3.06 GHz/GeForce GT130. while it was running Windows, it was benchmark to other PC models including the previous high-end iMac.
So, is the new iMac faster than the previous model? Yes it is. Is it a true gain? It depends on your needs and on the applications you are using (do not forget the test was performed with Windows). As expected the GeForce GT130 is faster than the GeForce 880GS powering the previous model by a clear but not dramatic margin. The same is true if you are using Photoshop, as the new model can be almost 50% faster depending on the task/filter.
For additional information:
PC Mag article