Only the 24" iMac has this feature right now and it (reportedly) uses an "MXM" card. MXM is Nvidia's mobile PCI Express implementation. Yes, the card is technically upgradeable. No it is not as simple as a desktop card swap. MXM cards are few and far between and most likely the two versions available for the 24" iMac are the only two "Mac" compatible MXM cards to exist.
1/12/07 at 2:03pm