PC100 SDRAM required, your iMac will accommodate 1 Gb with a matched pair. I read somewhere that PC133 SDRAM was used successfully, but speed increase was minimal.
Originally Posted by The Mac
Shop around on your local internet auction site. In addition to the brands already suggested, I've used Kingston successfully, and it's quite often to be found sold 2-hand. I've not yet encountered a problem with used RAM.
G4 eMac 1.25 GHz, OS 10.4.11
G3 iMac 600, OS 10.3.9
G3 b/w 350, dual drives, dual OS, 10.3.9, 9.2.2
Numerous peripheral gadgets, all working.