itinerary among roman sites in provence?

All these places are in easy reach of each other by car, and it really doesn't matter how you sequence them. Of the places you mentioned, Orange is the furthest from Arles, and even it is less than an hour's drive away via the motorway.

I've stayed in the city center of Arles and took day trips everywhere by car. As Tom said, it doesn't matter what order as they are all very close by. We made our decision where to go based on what was open that day and what the weather was like. Arles also has a very convenient train station too if you don't feel like driving one day. I would let opening days of the sites you want to see dictate your itenerary. Be sure to check on those as they seem to be a little unusual compared to most places and they change frequently. We went to Glanum one day and it was closed even though all the guide books listed it as open that day. It wasn't a big deal though as everything is so close, we went back a couple of days later.

On the way from Carcassonne you could visit Oppidum d'Ensérune, an ancient hill-town which Greeks and Romans occupied. It's near Capestang which is a nice Canal du Midi town. Rather than going to Pont du Gard the first day, I suggest you travel to Arles via the Camargue visiting Aigues Mortes and Saintes-Maries-de-la-Mer. With luck you'll see flamingos, white horses and the famous black bulls. You could tour Arles and Nimes one day, the Pont du Gard, Uzes and Orange another day, and Les Baux and Avignon another. Also fit in a stop at Châteauneuf-du-Pape if you're wine lovers. As you are interested in Roman sites, check out Le Pont Julien, a 3rd century bridge near Bonnieux in the Luberon. It was, rather amazingly, in use until 1995. Today it can be walked across. The Michelin Green Guides for Languedoc-Roussillon and for Provence have more details and recommended drives.