What makes the most sense is probably to get a SIM in Italy, run out most of the minutes, and then one in France. I'm not concerned with paying for a second SIM and any unused minutes--small percentage of the cost of the trip
--but I just want to have my cell phone is my primary means of communication.
Of course, it would be easier to have one phone number throughout the trip rather than two, but that means either buying an international SIM or just keeping the Italian one while in France. Either way, still much cheaper than just using Cingular's international roaming.