Good news for 24-inch iMac owners. Your computer can spit out the stock graphics card in favor of a new one, if you so desire. Thanks to the iMac's MXM PCI Express connector, you can go out and buy an upgrade for your computer, ensuring your Mac gaming (or Boot Camp Windows gaming) is always up to date.
MXM is a notebook design effort that allows Mobile PCI Express cards to be consistent across laptop manufacturers. That means easier production for card makers and the ability to upgrade the cards for the laptop owners.
Graphics card in 24-inch iMac is apparently upgradeable [MacsimumNews]