CMS 4GB (1X4GB) DDR2 6400 800MHZ Non ECC SODIMM Memory Ram Upgrade Compatible with Apple® iMac Core 2 Duo 2.4 20-Inch Early 2008 - A42 at Amazon.com
How to Upgrade iMac RAM (20" and 24" Aluminum): EveryMac.com
Memory Upgrades for Apple iMac 2008 - Up to 6GB
A-Tech for Apple 1GB Module PC2-5300 667MHz Mac Mini iMac MacBook Mid 2007 Late 2008 Late 2006 MB466LL/A A1278 MB467LL/A A1208 MA589LL A1207 MA456LL A1200 MB138LL/A A1176 MB139LL/A Memory RAM at Amazon.com
4GB Kit Apple iMac (April 2008) Memory Upgrade 2 x 2GB (MB413G/A) DDR2 PC2-6400 SODIMMs at Amazon.com
parts-quick 6GB Kit (4GB + 2GB) Memory for iMac 2008 Apple MacBook 2009 PC2-6400 800MHz DDR2 SODIMM at Amazon.com