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DDR SDRAM v. PC-100 or PC-1333 SDRAM?
If you don't know which version you have, please consult our SDRAM vs. DDR SDRAM guide.

In early 2003, Apple upgraded the iMac Flat Panel's memory controller to run high-speed DDR memory. Earlier models all run standard SDRAM. The DDR version can run a maximum of 2GB (two 1GB modules.)

User Slot:
The User Slot is the memory slot on the bottom of the machine. When you purchase your new iMac, this is the slot that will be empty. This slot uses a SO-DIMM.
User Slot RAM Installation Guide

Apple Slot:
Your new iMac will arrive with either a 128MB or a 256MB DIMM already installed. In order to upgrade the RAM in this Apple slot, you will need to remove the original RAM first. This slot uses a standard Long-DIMM.
Apple Slot RAM Installation Guide