I used to live in Paris and I worked at CDG. I do NOT recommend taking a taxi into town. If you run into a traffic jam, you wont be happy with the fee. I also ran into a lot of taxis who took me around town saying they didn't "understand" me. I studied in France before I moved there and was hired originally by the airlines as a French speaker. No one in any other aspect of my life here in France doesn't "understand" me.
I started giving directions, which not all were thrilled with but I made it clear that I would be doing this when I entered.
I used to take the Air France bus, which is more secure than the RER. Just watch out for pick pockets as freshly arrived tourists are prime targets. The Air France bus is just a big more money and the only passengers are those going to and from the airport. No gangs of kids getting on and off the various stations like you see on the RER to CDG.
Since France is on the Euro, try to get some before leaving. Ask around to any friends who have been anywhere in the Euro zone recently (Spain, Holland, Germany, etc.) so it shouldn't be hard.
I, personally don't think you need a Mastercard. Just Visa will do but let someone else chime in on that subject.
I have my money in two credit unions so I second that one, although it may not be an option for you.
I earned dollars when I lived in Paris so I had an American credit card. Be warned that they don't always work! Usually the contact couldn't be made. One card would not work and the other would (go figure...) Perhaps this has improved since I lived there because I now have a French "pouce" (chip) debit card.
One tip is to pull out a decent amount of money each time. Don't walk around with wads of cash but you will go through 200 Euros in just a few days.