We are going to be france -- 4 days in Paris, then down to Provence for two days before heading to Nice , then to Italy.
Our hotel is in aix-en-provence. Would it be better to spend the 2 or 3 days there, or is it possible to get to Arles or Avignon without wasting too much time traveling back and forth -- Also any suggestions about which places to cities to visit.
Aix is a poor base from which to see the sights of the Rhone valley.
Frankly, I would advise changing your plans and staying in Arles or Avignon.
Also, some transit options are seasonal, and you do not say when you are going, but Arles, Avignon, Nimes, and Orange are all on the train line.
I stayed in Nimes for several days last summer and daytripped to the places you mentioned.
Those towns are connected by SNCF trains. http://en.voyages-sncf.com/en/
I would prefer to be based in Avignon, too. It's bigger, giving more choices, and offers plenty of package day trips. The local tourist board is quite helpful: http://www.avignon-tourisme.com/home-1-2.html
I second basing yourself in Avignon. 2 days would not be enough if you wish to day trip too.
In the direction of Aix for one day, then in the direction of Arles/Nimes another day = spending at least 3 nights, preferably 4!
There is really that much to see and do (and better than Nice, IMO).