My 3.06 4850 runs unbearably hot to touch when I am playing WoW...
However, when I look at my iStat figures, the temps seem pretty normal to me with CPU and GPU temps averaging around 65.
I guess that the design of the iMac makes it a very good heatsink - just don't rest your hands on the thing when you're using it for 3d intensive stuff.
Therefore, with these figures in mind, I shouldn't be too worried about the heat generated from the 4850.
I hope that this is the answer you're looking for. I apologize if you do want numerical statistics for all the iMac graphic cards.