I have the 4GB and beam for my iMac 24-inch, ordered EARLY 2008, which according to Apple supports a total of only 4GB. But lo and behold, it works well with 2 + 4GB!
iMac 24-inch, early 2008 Processor 2.8 GHz Intel Core 2 Duo Memory 6 GB 800 MHz DDR2 SDRAM Video card NVIDIA GeForce 8800 GS 512 MB Software OS X 10.8