According to this MacRumors forum post, the 7600 GT in the 24" iMac runs at only 500 MHz core and 575 MHz memory. There could be a couple of reasons for this:
1) Apple cheaped out and is using cheaper components
2) Apple felt the heat generated from the GPU at higher clocks could compromise the iMac's reliability.
3) Apple felt the heat generated from the GPU at higher clocks would cause the fan to come on too often and make the iMac louder.
Whatever the reason, at least under Windows XP, one can clock it back to usual PC 7600 GT speeds. Whether that is safe or not is unknown at this time. It would be nice if someone could write such a utility for Mac OS X, to compare performance benchmarks with say Cinebench, Aperture, and 3D games.