Apple Mac A1312 27" iMac Aluminium Memory Cover Bottom Case RAM Access – Dealm4kers
4GB [2x2GB] DDR2-667 (PC2-5300) RAM Memory Upgrade Kit for The Apple iMac 7,1 (20-inch, 2.0GHz, MA876LL/A) Intel Core 2 Duo (Genuine A-Tech Brand) at Amazon.com
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET