To determine what kind of RAM (memory) is needed for your iMac select the model you have from the list below.

iMac G3 (233, 266, 333 MHz)
iMac G3 (350, 400, 500, 600 MHz)
iMac G4 (700, 800 MHz)
iMac G4 (1.00, 1.25 GHz)
iMac G5 (1.60, 1.80 GHz)
iMac G5 (1.9, 2.1 GHz)
iMac Intel Core Duo
iMac Intel Core 2 Duo (1.83 GHz)
iMac Intel Core 2 Duo (Late 2006)
iMac Intel Core 2 Duo (Mid 2007)
iMac Intel Core 2 Duo (Early 2008)
iMac Intel Core 2 Duo (Early 2009)

iMac G3 (233, 266, 333 MHz)
Maximum Memory: 512 MB
Memory Type: 144-pin SODIMM SDRAM PC-133 [buy]
Memory Slots: 2
Slot Maximum: 256 MB

iMac G3 (350, 400, 500, 600 MHz)
Maximum Memory: 1 GB
Memory Type: 168-pin DIMM SDRAM PC-133 [buy]
Memory Slots: 2
Slot Maximum: 512 MB

iMac G4 (700, 800 MHz)
Maximum Memory: 1 GB
Memory Type: 144-pin SODIMM SDRAM PC-133 [buy]
Memory Slots: 2
Slot Maximum: 512 MB

iMac G4 (1.00, 1.25 GHz)
Maximum Memory: 1 GB
Memory Type: 200-pin SODIMM DDR PC-2700 [buy]
Memory Slots: 1 (*Has DIMM slot that is not accessible)
Slot Maximum: 512 MB

iMac G5 (1.60, 1.80 GHz)
Maximum Memory: 2 GB
Memory Type: 184-pin DIMM DDR PC-3200 [buy]
Memory Slots: 2
Slot Maximum: 1 GB (Best to use pairs)

iMac G5 (1.9, 2.1 GHz)
Maximum Memory: 2.5 GB
Memory Type: 240-pin DIMM DDR2 PC2-5300 [buy]
Memory Slots: 1 (512 MB not removable)
Slot Maximum: 2 GB

iMac Intel Core Duo
Maximum Memory: 2 GB
Memory Type: 200-pin SODIMM DDR2 PC2-5300 [buy]
Memory Slots: 2
Slot Maximum: 1 GB

iMac Intel Core 2 Duo (1.83 GHz)
Maximum Memory: 2 GB
Memory Type: 200-pin SODIMM DDR2 PC2-5300 [buy]
Memory Slots: 2
Slot Maximum: 1 GB

iMac Intel Core 2 Duo (Late 2006)
Maximum Memory: 3 GB
Memory Type: 200-pin SODIMM DDR2 PC2-5300 [buy]
Memory Slots: 2
Slot Maximum: 2 GB

iMac Intel Core 2 Duo (Mid 2007)
Maximum Memory: 4 GB
Memory Type: 200-pin SODIMM DDR2 PC2-5300 [buy]
Memory Slots: 2
Slot Maximum: 2 GB

iMac Intel Core 2 Duo (Early 2008)
Maximum Memory: 4 GB
Memory Type: 200-pin SODIMM DDR2 PC2-6400 [buy]
Memory Slots: 2
Slot Maximum: 2 GB

iMac Intel Core 2 Duo (Early 2009)
Maximum Memory: 8 GB
Memory Type: 204-pin SODIMM DDR3 PC3-8500 [buy]
Memory Slots: 2
Slot Maximum: 4 GB