AMD’s graphics branch ATI has won one of the greatest victories in its eternal battle against Nvidia. The new, soon to be available iMac will only feature ATI graphics.
The reason behind this decision is most likely the fact that ATI is the only company that offers sub $149 graphics in DirectX 11 coating. Some might believe that Apple is upset with Nvidia’s big packaging issue, but the most logical reason is that Nvidia’s DirectX 11 cheaper parts are yet to come.
The new iMACs now come with Radeon HD 4670, the only DirectX 10.1 part in this selecton, followed by DirectX 11 HD 5670 and HD 5750 cards. Also, the new series is powered by Intel Core i3 and Core i5 processors rather than Core 2 Duos and quads.
The new 21.5-inch Apple iMac comes with Radeon HD 4670 with 256MB GDDR3 memory and Radeon HD 5670 with 512MB of GDDR3 memory, while the 27-inch model comes with a dual-core processor and Radeon HD 5670 with 512MB of DDR3 memory or Radeon HD 5750 with 1GB of GDDR5 memory.
Apple even claims on its website that “Games scream on iMac, thanks to advanced ATI graphics. The discrete ATI Radeon HD 4670 processor delivers faster performance compared to the integrated NVIDIA GeForce 9400M processor in the previous-generation iMac. That’ll keep you on your game, even inside the complex 3D environments of Call of Duty 4 or EVE. For faster graphics performance, choose an iMac with the ATI Radeon HD 5670 or ATI Radeon HD 5750.”
Not much needs to be said after reading this but you can clearly see that at this time there wont be any iMacs with Geforce inside, at least not in this generation. This is ceratainly a great win for ATI as Apple doesn't refresh its iMac lineup that often, so it's likely that there won't be any changes and that Nvidia is out of the iMac picture at least until next year.
You can find more details here.