I agree with your strategy. I live in Paris and I have a) a US BofA debit card that I use to access my US bank accounts; b) a Capital One card. Also have another card as a backup just in case.
RER is around 8.60 Euros per person one way. One recommendation is to always (always always always) take the express train into the city, even if it means waiting an extra 15 minutes. The express is the one that goes directly, or almost directly, to Gare du Nord from the airport. You can look up on the screen at the RER platform to see the stops that the train will make, and if there are a bunch of stops before Gare du Nord, then it is the local train. The express either goes directly to Gare du Nord, or it stops at Parc des Expositions first then direct to Gare du Nord.
Note, if you are arriving at Terminal 1, you will need to take the CDGVal to the RER station.