The original iMac G3 (Bondi Blue) took PC66 SODIMMs, and the 333 MHz iMac took PC100 SODIMMs. These are backwards compatible. So you can put PC100 SODIMMs in the 233 MHz iMac without too many issues. Both of these iMacs require a low profile SODIMM to be installed on the bottom to see more than 128MB.

Starting with the translucent casing(slot-loading CD), Apple switched from SODIMMs, to regular DIMMs. They also got away from the PC66 memory. They use PC100 or PC133 RAM. Also the RAM accessibility was much easier when trying to expand. There's just a door under the iMacs itself. All slot-loading iMacs with regular CRT displays all took the same RAM and are all expanded the same.

The iMac G4 took both SODIMMs and regular DIMMs. It just depends which slot you want to expand. That depends on how techie you are. The DIMM slot is inside the iMac G4 dome and requires professional installation. The 700/800 MHz iMac G4's both took PC133 RAM, even the first 17" iMac G4 required PC133 RAM. It wasn't until the 1GHz iMac G4 model that DDR RAM was used.

The iMac G5 at first used regular DDR RAM. It was really easy to expand too by just backing off 3 screws on the bottom of the iMac and the back came right off. If you have an iMac with an iSight Camera, you will need to use DDR2 RAM, which are SODIMMs. The remaining iMacs all use DDR2 SODIMMs, including the current Aluminum iMac.

In spring of 2009 Apple updated the iMac using a new NVIDIA chipset which provided suppport for DDR3 RAM. It also increased the max ram amount to 8GB. As with previous models, it still uses SODIMMs.

In the fall of 2009, Apple updated the iMacs with a new enclosure with new processor choices and wider screens. As a result, there are more RAM slots and the maximum RAM has been increased to 16GB. All models use the same PC8500 DDR3 RAM SODIMMs (non-ECC).

iMac

G3 Processor CRT


233/333 MHz G3 (Bondi Blue/5 Flavors)
PC66/100 SODIMM (notebook RAM)
Low Profile required for lower slot over 256MB in size.
Max RAM is 512MB (2 x 256)




400/500/600/700 MHz G3 (Slotloading/iMac DV/iMac DV SE/Dalmatian/CDRW)
PC100/133 RAM
Max RAM 1GB (2 x 512 MB)

G4 Processor Flat Panel


700/800 MHz G4 (iLamp)
PC133 SODIMM/PC133 DIMM (internal slot not user accessible)
Max RAM 1GB (2 x 512 MB)


1.0 GHz 17" G4 (iLamp)
PC2100 DDR RAM
Max RAM 2GB (2 x 1 GB)


1.0/1.25 GHz 15"/17"/20" G4 (USB 2.0)
PC2700 DDR RAM
Max RAM 2GB (2 x 1GB)

G5 Processor flat Panel


1.6/1.8 GHz G5
PC3200 DDR RAM
Max RAM 2GB (2 x 1GB)


1.9/2.1 GHz G5 (iSight Camera)
PC2-4200 DDR2 SODIMM RAM
Max RAM 2.5 GB

Intel Core Duo Processor Flat Panel


2.0/2.16 GHz Core Duo 17"/20"
PC2-5300 DDR2 SODIMM RAM
Max RAM 2.0 GB

Intel Core 2 Duo Processor Flat Panel


1.83/2.0/2.33 GHz Core 2 Duo 17"/20"/24"
PC2-5300 DDR2 SODIMM RAM
Max RAM 3.0 GB


2.0/2.4/2.8 GHz Core 2 Duo 20"/24" (Aluminum Enclosure)
PC2-5300 DDR2 SODIMM RAM
Max RAM 4.0 GB




2.4/2.66/2.8/3.06 GHz Core 2 Duo 20"/24" (Aluminum Enclosure Spring '08)
PC2-6400 DDR2 SODIMM RAM
Max RAM 4.0 GB


2.66/2.93/3.06 GHz Core 2 Duo 20"/24" (Aluminum Enclosure Spring '09)
PC3-8500 DDR3 SODIMM RAM
Max RAM 8.0 GB


3.06 GHz Core 2 Duo, 2.66 GHz Core i5, 2.8 GHz Core i7 21.5"/27" (Aluminum Enclosure Fall '09)
PC3-8500 DDR3 SODIMM RAM
Max RAM 16.0 GB