I think your best bet for this itinerary is the train; France's rail network is one of the best in the world, in my opinion. Check out www.raileurope.com, where you can find information about France-Italy rail passes. These give you unlimited access to rail services in both countries for a specified number of days within a two-month period. You can get to Rome from Nice, near Aix-en-Provence, by way of Milan. It's quite a long slog for one trip, so you may want to break things up with a stop in Genoa. It's also quite simple to then hop from Rome to Florence and Venice.