I'm not really an expert, but if I were doing phono, I would probably use the phono preamp in my receiver. Connect the turntable to the receiver as normal, and then use a tape monitor out or some other RCA output from the receiver into the iMac sound input. As long as you adjust the levels appropriately, this should not be a problem.
I would do this because the signal output from turntables is much lower than from other components (CD players, cassettes, etc.). Most receivers have a built-in phono preamp to bring the signal up before sending it through the main amplifier, and I would generally think this would provide better results than plugging straight into the iMac and cranking up the input levels. Good luck.