iMac 500 MHz uses PC100 RAM. eMac uses PC133 RAM.
i believe they are compatible, although PC100 RAM will obviously run only up to 100 MHz, even on a 133 MHz bus. it's possible that i'm mistaken in that PC133 will run on PC100 but not the other way around, which is what you are trying to do...
the best bet, obviously, is to try it...