How to Upgrade iMac RAM (20" and 24" Aluminum): EveryMac.com
Install memory in an iMac - Apple Support
install memory in iMac - mid 2007 model - YouTube
A-Tech for Apple 6GB Kit (1x2GB + 1x4GB) DDR2 667MHz PC2-5300 200-pin SODIMM for MacBook (Late 2007, Early 2008, Early 2009), MacBook Pro (Mid 2007, Early 2008), iMac (Mid 2007) - Memory RAM Upgrade at Amazon.com
New Memory Ram Upgrade 4 Apple iMac 24" Core 2 Duo 2.4GHZ 2.8GHz A1225 MID 2007 | eBay
MemoryMasters 6GB (4GB + 2GB Memory Upgrade Kit) Memory RAM for Apple iMac 2007 7,1 at Amazon.com
OWC 16GB (2x8GB) ram Upgrade PC3-12800 DDR3L 1600MHz sodimm 204 Pin CL11 Memory Upgrade Kit Compatible with Late iMac, Mac Mini, and MacBook Pro Computer at Amazon.com