It always depends on the model....
G3 iMac started out with so-dimms (laptop memory) PC66, then went ot full size PC100/133 desktop RAM.
G4 iMac started with so-dimm user serviceable/accessible up to 512MB or full size desktop RAM internally, both PC133 then all the way to PC2100 & PC2700 DDR RAM.
So again, it depends on the model...