The 2.1MB EFI update applies only to those users of 27-inch iMacs powered by the quad-core Intel Core i5 and Core i7 processors: it fixes a problem that caused the process to ramp up while playing audio through the headphone jack and corrects an issue that resulted in the display backlight not turning on when the iMac was powered on. It requires Mac OS X 10.6.3 or later.
The comparatively tiny 397KB SMC Firmware Update addresses compatibility issues with
Target Display Mode—that’s the feature that allows you to use an iMac as an external monitor. It also requires Mac OS X 10.6.3 or later.