Your iMac is either a G5 (PowerPC) or and Intel. They are different processors, so you have one or the other. All iMacs only have 2 RAM slots. In order to get more than 2 GB's into it you need to use 2 GB RAM sticks. They can be expensive for the G5, and I have not seen any for Intel. Two GB should be enough, though. Why do you want more?