New iMacs are going to use Nvidia graphics cards. But Nvidia didn´t have a suitable graphics card for the high end 27" iMac. Apple is going to use a new Nvidia mid range chip called the GK106 which has been rumoured to be released in the coming weeks. See the correlation with the new iMac rumours? Tweaktown already has a review of this chip in the form of a Desktop graphics card the GTX660 non TI you can find it in the review section of the site. It might not be a GTX680m and i don´t know what mobile parts are going to be based on it but it´s still a powerful card!