20-Inch iMac (2007-2008) Hard Drive Installation Video (iMac7,1 iMac8,1) - YouTube
How to Upgrade iMac RAM (20" and 24" Aluminum): EveryMac.com
Install memory in an iMac - Apple Support
MemoryMasters 6GB (4GB + 2GB Memory Upgrade Kit) Memory RAM for Apple iMac 2007 7,1 at Amazon.com
OWC 4GB PC5300 DDR2 667MHz SO-DIMM Memory RAM Upgrade Compatible with MacBook (Late 2007/Early 2008), MacBook Pro (Mid/Late 2007, Early 2008), & iMac (Late 2007) with Core 2 Duo CPU (OWC5300DDR2S4GB) at Amazon.com