Do I need to purchase a French-Spanish rail pass if I'm traveling from Avignon to Barcelona? Most of the trip will be in France. We will leave Barcelona and return to Paris. Some have told me to buy the Single French pass and pay the extra for the trip into Spain.
If you're getting a France pass for a few trips within that country, then the seat reservation for direct TGV trains to/from Barcelona ($38 in 2nd class or $49 in 1st class) covers the portion within Spain. You don't need Spain on the pass. See also http://www.ricksteves.com/travel-tips/transportation/trains/france-rail-passes. You can add TGV seat reservations in the same online order with the railpass, which is a good idea since the number of places for passholders is limited.
First, I suspect that you will be arriving in Paris before traveling to Avignon. You don't indicate when this trip will take place but if you are willing to accept a non-refundable ticket and you still have 90 days before your travel so that you can get the lowest discount fares, there is a great high-speed TGV run from Paris to either Avignon-TGV or Avignon Centre. Trip takes 2hrs39min. Fares are as low as 25Euro but you'll need a bit of luck to score those tickets. Still there are some great bargains. Avignon to Barcelona you can do for as low as 49Euro. Trip takes 3hrs41min.
Two ways to get back to Paris from Barcelona. You can fly on www.vueling.com to Paris-Orly for as low as 59.99Euro. (Vueling will not accept your Eurail pass). Or, you can take the new Barcelona/Paris day train. 6hrs36min. No train changes. Fares are as low as 59.99Euro.
For what you are planning, you really don't need any kind of railpass. You can do all of this train travel for as low as 160Euro. True tickets will be non-refundable but if you plan well, it works great. We do it all the time. Note that you will pay a 9Euro supplemental fee when you ride the TGVs with a Eurail pass within France. For the Barcelona/Paris train, two big ouches. 1) the supplemental fee for passholders on this really new day train is 25.60Euros for a 2nd class ticket. This fee is shown on the Eurail website - ouch. 2) You must have a valid pass for both Spain and France - double ouch. Also note that the French limit their seats to Eurail passholders. I've never seen a report where this was enforced but its on the Eurail website.
You can book all of your tickets online at http://en.voyages-sncf.com Just don't select USA as your Ticket Collection Country. If you do, the SNCF system will attempt to switch you to RailEurope which you do not want. Just select France. No the website will not be presented in French. You'll see English. Should you purchase online, you can select E-Ticket for most TGV runs and print out your tickets within minutes after your purchase.