not sure if you have visited forum.thesim3.com mac section, if not i would recommend you pay it a visit.
i asked the same thing over there and there was one user that has a 21" (i forget her specs), but she said she had no lagging and the graphics were all on high. she was quite surprised at how drastic the gaming experience was from her mbp. from what i've heard simming through bootcamp is 10x better, but i just can't bring myself to not use the native os.
now to bring balance to the picture there are several very serious errors caused playing the game on mac (many are game ruining ie crashing within a few minutes of play, etc) so i would be careful or at least ready to play via bootcamp if you do play on the imac.